![Fengguang Wu](/assets/img/avatar_default.png)
Share the same page flag bit for PG_readahead and PG_reclaim. One is used only on file reads, another is only for emergency writes. One is used mostly for fresh/young pages, another is for old pages. Combinations of possible interactions are: a) clear PG_reclaim => implicit clear of PG_readahead it will delay an asynchronous readahead into a synchronous one it actually does _good_ for readahead: the pages will be reclaimed soon, it's readahead thrashing! in this case, synchronous readahead makes more sense. b) clear PG_readahead => implicit clear of PG_reclaim one(and only one) page will not be reclaimed in time it can be avoided by checking PageWriteback(page) in readahead first c) set PG_reclaim => implicit set of PG_readahead will confuse readahead and make it restart the size rampup process it's a trivial problem, and can mostly be avoided by checking PageWriteback(page) first in readahead d) set PG_readahead => implicit set of PG_reclaim PG_readahead will never be set on already cached pages. PG_reclaim will always be cleared on dirtying a page. so not a problem. In summary, a) we get better behavior b,d) possible interactions can be avoided c) racy condition exists that might affect readahead, but the chance is _really_ low, and the hurt on readahead is trivial. Compound pages also use PG_reclaim, but for now they do not interact with reclaim/readahead code. Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn> Cc: Rusty Russell <rusty@rustcorp.com.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
471 lines
13 KiB
C
471 lines
13 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/task_io_accounting_ops.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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/*
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* Convienent macros for min/max read-ahead pages.
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* Note that MAX_RA_PAGES is rounded down, while MIN_RA_PAGES is rounded up.
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* The latter is necessary for systems with large page size(i.e. 64k).
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*/
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#define MAX_RA_PAGES (VM_MAX_READAHEAD*1024 / PAGE_CACHE_SIZE)
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#define MIN_RA_PAGES DIV_ROUND_UP(VM_MIN_READAHEAD*1024, PAGE_CACHE_SIZE)
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struct backing_dev_info default_backing_dev_info = {
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.ra_pages = MAX_RA_PAGES,
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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_index = -1;
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}
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EXPORT_SYMBOL_GPL(file_ra_state_init);
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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 & 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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put_pages_list(pages);
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break;
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}
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task_io_account_read(PAGE_CACHE_SIZE);
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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;
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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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/* Clean up the remaining pages */
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put_pages_list(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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pagevec_lru_add(&lru_pvec);
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ret = 0;
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out:
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return ret;
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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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pgoff_t offset, unsigned long nr_to_read,
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unsigned long lookahead_size)
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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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pgoff_t 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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if (page_idx == nr_to_read - lookahead_size)
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SetPageReadahead(page);
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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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pgoff_t 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, 0);
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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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* 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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pgoff_t 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, 0);
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}
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/*
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* Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
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* sensible upper limit.
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*/
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unsigned long max_sane_readahead(unsigned long nr)
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{
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return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE)
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+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
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}
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/*
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* Submit IO for the read-ahead request in file_ra_state.
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*/
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unsigned long ra_submit(struct file_ra_state *ra,
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struct address_space *mapping, struct file *filp)
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{
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unsigned long ra_size;
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unsigned long la_size;
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int actual;
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ra_size = ra_readahead_size(ra);
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la_size = ra_lookahead_size(ra);
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actual = __do_page_cache_readahead(mapping, filp,
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ra->ra_index, ra_size, la_size);
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return actual;
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}
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EXPORT_SYMBOL_GPL(ra_submit);
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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 / 32)
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newsize = newsize * 4;
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else if (newsize <= max / 4)
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newsize = newsize * 2;
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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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* Get the previous window size, ramp it up, and
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* return it as the new window size.
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*/
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static unsigned long get_next_ra_size(struct file_ra_state *ra,
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unsigned long max)
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{
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unsigned long cur = ra->readahead_index - ra->ra_index;
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unsigned long newsize;
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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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return min(newsize, max);
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}
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/*
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* On-demand 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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* |-------- last readahead window -------->|
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* |-- application walking here -->|
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* ======#============|==================#=====================|
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* ^la_index ^ra_index ^lookahead_index ^readahead_index
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*
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* [ra_index, readahead_index) represents the last readahead window.
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*
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* [la_index, lookahead_index] is where the application would be walking(in
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* the common case of cache-cold sequential reads): the last window was
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* established when the application was at la_index, and the next window will
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* be bring in when the application reaches lookahead_index.
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*
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* To overlap application thinking time and disk I/O time, we do
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* `readahead pipelining': Do not wait until the application consumed all
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* readahead pages and stalled on the missing page at readahead_index;
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* Instead, submit an asynchronous readahead I/O as early as the application
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* reads on the page at lookahead_index. Normally lookahead_index will be
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* equal to ra_index, for maximum pipelining.
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*
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* In interleaved sequential reads, concurrent streams on the same fd can
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* be invalidating each other's readahead state. So we flag the new readahead
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* page at lookahead_index with PG_readahead, and use it as readahead
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* indicator. The flag won't be set on already cached pages, to avoid the
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* readahead-for-nothing fuss, saving pointless page cache lookups.
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*
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* prev_index tracks the last visited page in the _previous_ read request.
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* It should be maintained by the caller, and will be used for detecting
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* small random reads. Note that the readahead algorithm checks loosely
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* for sequential patterns. Hence interleaved reads might be served as
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* sequential ones.
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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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* The code ramps up the readahead size aggressively at first, but slow down as
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* it approaches max_readhead.
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*/
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/*
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* A minimal readahead algorithm for trivial sequential/random reads.
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*/
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static unsigned long
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ondemand_readahead(struct address_space *mapping,
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struct file_ra_state *ra, struct file *filp,
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struct page *page, pgoff_t offset,
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unsigned long req_size)
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{
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unsigned long max; /* max readahead pages */
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pgoff_t ra_index; /* readahead index */
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unsigned long ra_size; /* readahead size */
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unsigned long la_size; /* lookahead size */
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int sequential;
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max = ra->ra_pages;
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sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);
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/*
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* Lookahead/readahead hit, assume sequential access.
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* Ramp up sizes, and push forward the readahead window.
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*/
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if (offset && (offset == ra->lookahead_index ||
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offset == ra->readahead_index)) {
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ra_index = ra->readahead_index;
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ra_size = get_next_ra_size(ra, max);
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la_size = ra_size;
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goto fill_ra;
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}
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/*
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* Standalone, small read.
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* Read as is, and do not pollute the readahead state.
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*/
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if (!page && !sequential) {
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return __do_page_cache_readahead(mapping, filp,
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offset, req_size, 0);
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}
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/*
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* It may be one of
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* - first read on start of file
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* - sequential cache miss
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* - oversize random read
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* Start readahead for it.
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*/
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ra_index = offset;
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ra_size = get_init_ra_size(req_size, max);
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la_size = ra_size > req_size ? ra_size - req_size : ra_size;
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/*
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* Hit on a lookahead page without valid readahead state.
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* E.g. interleaved reads.
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* Not knowing its readahead pos/size, bet on the minimal possible one.
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*/
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if (page) {
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ra_index++;
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ra_size = min(4 * ra_size, max);
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}
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fill_ra:
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ra_set_index(ra, offset, ra_index);
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ra_set_size(ra, ra_size, la_size);
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return ra_submit(ra, mapping, filp);
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}
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/**
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* page_cache_readahead_ondemand - generic file readahead
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* @mapping: address_space which holds the pagecache and I/O vectors
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* @ra: file_ra_state which holds the readahead state
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* @filp: passed on to ->readpage() and ->readpages()
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* @page: the page at @offset, or NULL if non-present
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* @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
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* @req_size: hint: total size of the read which the caller is performing in
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* PAGE_CACHE_SIZE units
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*
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* page_cache_readahead_ondemand() is the entry point of readahead logic.
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* This function should be called when it is time to perform readahead:
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* 1) @page == NULL
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* A cache miss happened, time for synchronous readahead.
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* 2) @page != NULL && PageReadahead(@page)
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* A look-ahead hit occured, time for asynchronous readahead.
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*/
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unsigned long
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page_cache_readahead_ondemand(struct address_space *mapping,
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struct file_ra_state *ra, struct file *filp,
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struct page *page, pgoff_t offset,
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unsigned long req_size)
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{
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/* no read-ahead */
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if (!ra->ra_pages)
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return 0;
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if (page) {
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/*
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* It can be PG_reclaim.
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*/
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if (PageWriteback(page))
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return 0;
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ClearPageReadahead(page);
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/*
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* Defer asynchronous read-ahead on IO congestion.
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*/
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if (bdi_read_congested(mapping->backing_dev_info))
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return 0;
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}
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/* do read-ahead */
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return ondemand_readahead(mapping, ra, filp, page,
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offset, req_size);
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}
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EXPORT_SYMBOL_GPL(page_cache_readahead_ondemand);
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