1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
558 lines
16 KiB
C
558 lines
16 KiB
C
/*
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* mm/readahead.c - address_space-level file readahead.
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*
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* Copyright (C) 2002, Linus Torvalds
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*
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* 09Apr2002 akpm@zip.com.au
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* Initial version.
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*/
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#include <linux/kernel.h>
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#include <linux/fs.h>
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <linux/blkdev.h>
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#include <linux/backing-dev.h>
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#include <linux/pagevec.h>
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void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
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{
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}
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EXPORT_SYMBOL(default_unplug_io_fn);
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struct backing_dev_info default_backing_dev_info = {
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.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE,
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.state = 0,
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.capabilities = BDI_CAP_MAP_COPY,
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.unplug_io_fn = default_unplug_io_fn,
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};
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EXPORT_SYMBOL_GPL(default_backing_dev_info);
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/*
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* Initialise a struct file's readahead state. Assumes that the caller has
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* memset *ra to zero.
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*/
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void
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file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
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{
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ra->ra_pages = mapping->backing_dev_info->ra_pages;
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ra->prev_page = -1;
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}
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/*
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* Return max readahead size for this inode in number-of-pages.
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*/
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static inline unsigned long get_max_readahead(struct file_ra_state *ra)
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{
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return ra->ra_pages;
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}
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static inline unsigned long get_min_readahead(struct file_ra_state *ra)
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{
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return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
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}
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static inline void ra_off(struct file_ra_state *ra)
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{
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ra->start = 0;
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ra->flags = 0;
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ra->size = 0;
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ra->ahead_start = 0;
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ra->ahead_size = 0;
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return;
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}
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/*
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* Set the initial window size, round to next power of 2 and square
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* for small size, x 4 for medium, and x 2 for large
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* for 128k (32 page) max ra
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* 1-8 page = 32k initial, > 8 page = 128k initial
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*/
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static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
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{
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unsigned long newsize = roundup_pow_of_two(size);
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if (newsize <= max / 64)
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newsize = newsize * newsize;
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else if (newsize <= max / 4)
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newsize = max / 4;
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else
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newsize = max;
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return newsize;
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}
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/*
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* Set the new window size, this is called only when I/O is to be submitted,
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* not for each call to readahead. If a cache miss occured, reduce next I/O
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* size, else increase depending on how close to max we are.
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*/
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static inline unsigned long get_next_ra_size(struct file_ra_state *ra)
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{
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unsigned long max = get_max_readahead(ra);
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unsigned long min = get_min_readahead(ra);
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unsigned long cur = ra->size;
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unsigned long newsize;
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if (ra->flags & RA_FLAG_MISS) {
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ra->flags &= ~RA_FLAG_MISS;
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newsize = max((cur - 2), min);
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} else if (cur < max / 16) {
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newsize = 4 * cur;
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} else {
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newsize = 2 * cur;
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}
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return min(newsize, max);
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}
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#define list_to_page(head) (list_entry((head)->prev, struct page, lru))
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/**
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* read_cache_pages - populate an address space with some pages, and
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* start reads against them.
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* @mapping: the address_space
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* @pages: The address of a list_head which contains the target pages. These
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* pages have their ->index populated and are otherwise uninitialised.
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* @filler: callback routine for filling a single page.
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* @data: private data for the callback routine.
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*
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* Hides the details of the LRU cache etc from the filesystems.
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*/
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int read_cache_pages(struct address_space *mapping, struct list_head *pages,
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int (*filler)(void *, struct page *), void *data)
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{
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struct page *page;
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struct pagevec lru_pvec;
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int ret = 0;
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pagevec_init(&lru_pvec, 0);
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while (!list_empty(pages)) {
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page = list_to_page(pages);
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list_del(&page->lru);
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if (add_to_page_cache(page, mapping, page->index, GFP_KERNEL)) {
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page_cache_release(page);
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continue;
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}
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ret = filler(data, page);
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if (!pagevec_add(&lru_pvec, page))
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__pagevec_lru_add(&lru_pvec);
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if (ret) {
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while (!list_empty(pages)) {
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struct page *victim;
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victim = list_to_page(pages);
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list_del(&victim->lru);
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page_cache_release(victim);
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}
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break;
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}
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}
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pagevec_lru_add(&lru_pvec);
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return ret;
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}
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EXPORT_SYMBOL(read_cache_pages);
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static int read_pages(struct address_space *mapping, struct file *filp,
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struct list_head *pages, unsigned nr_pages)
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{
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unsigned page_idx;
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struct pagevec lru_pvec;
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int ret = 0;
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if (mapping->a_ops->readpages) {
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ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
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goto out;
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}
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pagevec_init(&lru_pvec, 0);
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for (page_idx = 0; page_idx < nr_pages; page_idx++) {
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struct page *page = list_to_page(pages);
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list_del(&page->lru);
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if (!add_to_page_cache(page, mapping,
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page->index, GFP_KERNEL)) {
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mapping->a_ops->readpage(filp, page);
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if (!pagevec_add(&lru_pvec, page))
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__pagevec_lru_add(&lru_pvec);
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} else {
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page_cache_release(page);
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}
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}
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pagevec_lru_add(&lru_pvec);
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out:
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return ret;
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}
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/*
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* Readahead design.
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*
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* The fields in struct file_ra_state represent the most-recently-executed
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* readahead attempt:
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*
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* start: Page index at which we started the readahead
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* size: Number of pages in that read
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* Together, these form the "current window".
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* Together, start and size represent the `readahead window'.
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* prev_page: The page which the readahead algorithm most-recently inspected.
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* It is mainly used to detect sequential file reading.
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* If page_cache_readahead sees that it is again being called for
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* a page which it just looked at, it can return immediately without
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* making any state changes.
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* ahead_start,
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* ahead_size: Together, these form the "ahead window".
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* ra_pages: The externally controlled max readahead for this fd.
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*
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* When readahead is in the off state (size == 0), readahead is disabled.
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* In this state, prev_page is used to detect the resumption of sequential I/O.
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*
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* The readahead code manages two windows - the "current" and the "ahead"
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* windows. The intent is that while the application is walking the pages
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* in the current window, I/O is underway on the ahead window. When the
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* current window is fully traversed, it is replaced by the ahead window
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* and the ahead window is invalidated. When this copying happens, the
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* new current window's pages are probably still locked. So
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* we submit a new batch of I/O immediately, creating a new ahead window.
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*
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* So:
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*
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* ----|----------------|----------------|-----
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* ^start ^start+size
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* ^ahead_start ^ahead_start+ahead_size
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*
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* ^ When this page is read, we submit I/O for the
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* ahead window.
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*
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* A `readahead hit' occurs when a read request is made against a page which is
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* the next sequential page. Ahead window calculations are done only when it
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* is time to submit a new IO. The code ramps up the size agressively at first,
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* but slow down as it approaches max_readhead.
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*
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* Any seek/ramdom IO will result in readahead being turned off. It will resume
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* at the first sequential access.
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*
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* There is a special-case: if the first page which the application tries to
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* read happens to be the first page of the file, it is assumed that a linear
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* read is about to happen and the window is immediately set to the initial size
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* based on I/O request size and the max_readahead.
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*
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* This function is to be called for every read request, rather than when
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* it is time to perform readahead. It is called only once for the entire I/O
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* regardless of size unless readahead is unable to start enough I/O to satisfy
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* the request (I/O request > max_readahead).
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*/
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/*
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* do_page_cache_readahead actually reads a chunk of disk. It allocates all
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* the pages first, then submits them all for I/O. This avoids the very bad
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* behaviour which would occur if page allocations are causing VM writeback.
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* We really don't want to intermingle reads and writes like that.
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*
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* Returns the number of pages requested, or the maximum amount of I/O allowed.
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*
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* do_page_cache_readahead() returns -1 if it encountered request queue
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* congestion.
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*/
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static int
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__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
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unsigned long offset, unsigned long nr_to_read)
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{
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struct inode *inode = mapping->host;
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struct page *page;
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unsigned long end_index; /* The last page we want to read */
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LIST_HEAD(page_pool);
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int page_idx;
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int ret = 0;
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loff_t isize = i_size_read(inode);
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if (isize == 0)
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goto out;
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end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
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/*
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* Preallocate as many pages as we will need.
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*/
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read_lock_irq(&mapping->tree_lock);
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for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
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unsigned long page_offset = offset + page_idx;
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if (page_offset > end_index)
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break;
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page = radix_tree_lookup(&mapping->page_tree, page_offset);
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if (page)
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continue;
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read_unlock_irq(&mapping->tree_lock);
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page = page_cache_alloc_cold(mapping);
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read_lock_irq(&mapping->tree_lock);
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if (!page)
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break;
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page->index = page_offset;
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list_add(&page->lru, &page_pool);
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ret++;
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}
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read_unlock_irq(&mapping->tree_lock);
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/*
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* Now start the IO. We ignore I/O errors - if the page is not
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* uptodate then the caller will launch readpage again, and
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* will then handle the error.
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*/
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if (ret)
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read_pages(mapping, filp, &page_pool, ret);
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BUG_ON(!list_empty(&page_pool));
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out:
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return ret;
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}
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/*
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* Chunk the readahead into 2 megabyte units, so that we don't pin too much
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* memory at once.
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*/
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int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
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unsigned long offset, unsigned long nr_to_read)
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{
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int ret = 0;
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if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
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return -EINVAL;
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while (nr_to_read) {
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int err;
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unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
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if (this_chunk > nr_to_read)
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this_chunk = nr_to_read;
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err = __do_page_cache_readahead(mapping, filp,
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offset, this_chunk);
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if (err < 0) {
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ret = err;
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break;
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}
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ret += err;
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offset += this_chunk;
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nr_to_read -= this_chunk;
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}
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return ret;
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}
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/*
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* Check how effective readahead is being. If the amount of started IO is
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* less than expected then the file is partly or fully in pagecache and
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* readahead isn't helping.
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*
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*/
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static inline int check_ra_success(struct file_ra_state *ra,
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unsigned long nr_to_read, unsigned long actual)
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{
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if (actual == 0) {
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ra->cache_hit += nr_to_read;
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if (ra->cache_hit >= VM_MAX_CACHE_HIT) {
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ra_off(ra);
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ra->flags |= RA_FLAG_INCACHE;
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return 0;
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}
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} else {
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ra->cache_hit=0;
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}
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return 1;
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}
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/*
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* This version skips the IO if the queue is read-congested, and will tell the
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* block layer to abandon the readahead if request allocation would block.
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*
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* force_page_cache_readahead() will ignore queue congestion and will block on
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* request queues.
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*/
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int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
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unsigned long offset, unsigned long nr_to_read)
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{
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if (bdi_read_congested(mapping->backing_dev_info))
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return -1;
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return __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
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}
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/*
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* Read 'nr_to_read' pages starting at page 'offset'. If the flag 'block'
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* is set wait till the read completes. Otherwise attempt to read without
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* blocking.
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* Returns 1 meaning 'success' if read is succesfull without switching off
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* readhaead mode. Otherwise return failure.
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*/
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static int
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blockable_page_cache_readahead(struct address_space *mapping, struct file *filp,
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unsigned long offset, unsigned long nr_to_read,
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struct file_ra_state *ra, int block)
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{
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int actual;
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if (!block && bdi_read_congested(mapping->backing_dev_info))
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return 0;
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actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
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return check_ra_success(ra, nr_to_read, actual);
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}
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static int make_ahead_window(struct address_space *mapping, struct file *filp,
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struct file_ra_state *ra, int force)
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{
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int block, ret;
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ra->ahead_size = get_next_ra_size(ra);
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ra->ahead_start = ra->start + ra->size;
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block = force || (ra->prev_page >= ra->ahead_start);
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ret = blockable_page_cache_readahead(mapping, filp,
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ra->ahead_start, ra->ahead_size, ra, block);
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if (!ret && !force) {
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/* A read failure in blocking mode, implies pages are
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* all cached. So we can safely assume we have taken
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* care of all the pages requested in this call.
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* A read failure in non-blocking mode, implies we are
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* reading more pages than requested in this call. So
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* we safely assume we have taken care of all the pages
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* requested in this call.
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*
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* Just reset the ahead window in case we failed due to
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* congestion. The ahead window will any way be closed
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* in case we failed due to excessive page cache hits.
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*/
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ra->ahead_start = 0;
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ra->ahead_size = 0;
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}
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return ret;
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}
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/*
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* page_cache_readahead is the main function. If performs the adaptive
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* readahead window size management and submits the readahead I/O.
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*/
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unsigned long
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page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
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struct file *filp, unsigned long offset,
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unsigned long req_size)
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{
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unsigned long max, newsize;
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int sequential;
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/*
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* We avoid doing extra work and bogusly perturbing the readahead
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* window expansion logic.
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*/
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if (offset == ra->prev_page && --req_size)
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++offset;
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/* Note that prev_page == -1 if it is a first read */
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sequential = (offset == ra->prev_page + 1);
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ra->prev_page = offset;
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max = get_max_readahead(ra);
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newsize = min(req_size, max);
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/* No readahead or sub-page sized read or file already in cache */
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if (newsize == 0 || (ra->flags & RA_FLAG_INCACHE))
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goto out;
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ra->prev_page += newsize - 1;
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/*
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* Special case - first read at start of file. We'll assume it's
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* a whole-file read and grow the window fast. Or detect first
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* sequential access
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*/
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if (sequential && ra->size == 0) {
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ra->size = get_init_ra_size(newsize, max);
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ra->start = offset;
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if (!blockable_page_cache_readahead(mapping, filp, offset,
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ra->size, ra, 1))
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goto out;
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/*
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* If the request size is larger than our max readahead, we
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* at least want to be sure that we get 2 IOs in flight and
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* we know that we will definitly need the new I/O.
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* once we do this, subsequent calls should be able to overlap
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* IOs,* thus preventing stalls. so issue the ahead window
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* immediately.
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*/
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if (req_size >= max)
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make_ahead_window(mapping, filp, ra, 1);
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goto out;
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}
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/*
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* Now handle the random case:
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* partial page reads and first access were handled above,
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* so this must be the next page otherwise it is random
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*/
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if (!sequential) {
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ra_off(ra);
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blockable_page_cache_readahead(mapping, filp, offset,
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newsize, ra, 1);
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goto out;
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}
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/*
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* If we get here we are doing sequential IO and this was not the first
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* occurence (ie we have an existing window)
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*/
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if (ra->ahead_start == 0) { /* no ahead window yet */
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if (!make_ahead_window(mapping, filp, ra, 0))
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goto out;
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}
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/*
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* Already have an ahead window, check if we crossed into it.
|
|
* If so, shift windows and issue a new ahead window.
|
|
* Only return the #pages that are in the current window, so that
|
|
* we get called back on the first page of the ahead window which
|
|
* will allow us to submit more IO.
|
|
*/
|
|
if (ra->prev_page >= ra->ahead_start) {
|
|
ra->start = ra->ahead_start;
|
|
ra->size = ra->ahead_size;
|
|
make_ahead_window(mapping, filp, ra, 0);
|
|
}
|
|
|
|
out:
|
|
return ra->prev_page + 1;
|
|
}
|
|
|
|
/*
|
|
* handle_ra_miss() is called when it is known that a page which should have
|
|
* been present in the pagecache (we just did some readahead there) was in fact
|
|
* not found. This will happen if it was evicted by the VM (readahead
|
|
* thrashing)
|
|
*
|
|
* Turn on the cache miss flag in the RA struct, this will cause the RA code
|
|
* to reduce the RA size on the next read.
|
|
*/
|
|
void handle_ra_miss(struct address_space *mapping,
|
|
struct file_ra_state *ra, pgoff_t offset)
|
|
{
|
|
ra->flags |= RA_FLAG_MISS;
|
|
ra->flags &= ~RA_FLAG_INCACHE;
|
|
}
|
|
|
|
/*
|
|
* Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
|
|
* sensible upper limit.
|
|
*/
|
|
unsigned long max_sane_readahead(unsigned long nr)
|
|
{
|
|
unsigned long active;
|
|
unsigned long inactive;
|
|
unsigned long free;
|
|
|
|
__get_zone_counts(&active, &inactive, &free, NODE_DATA(numa_node_id()));
|
|
return min(nr, (inactive + free) / 2);
|
|
}
|