/* * mm/readahead.c - address_space-level file readahead. * * Copyright (C) 2002, Linus Torvalds * * 09Apr2002 Andrew Morton * Initial version. */ #include <linux/kernel.h> #include <linux/fs.h> #include <linux/mm.h> #include <linux/module.h> #include <linux/blkdev.h> #include <linux/backing-dev.h> #include <linux/task_io_accounting_ops.h> #include <linux/pagevec.h> #include <linux/pagemap.h> void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page) { } EXPORT_SYMBOL(default_unplug_io_fn); struct backing_dev_info default_backing_dev_info = { .ra_pages = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE, .state = 0, .capabilities = BDI_CAP_MAP_COPY, .unplug_io_fn = default_unplug_io_fn, }; EXPORT_SYMBOL_GPL(default_backing_dev_info); /* * Initialise a struct file's readahead state. Assumes that the caller has * memset *ra to zero. */ void file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping) { ra->ra_pages = mapping->backing_dev_info->ra_pages; ra->prev_pos = -1; } EXPORT_SYMBOL_GPL(file_ra_state_init); #define list_to_page(head) (list_entry((head)->prev, struct page, lru)) /** * read_cache_pages - populate an address space with some pages & start reads against them * @mapping: the address_space * @pages: The address of a list_head which contains the target pages. These * pages have their ->index populated and are otherwise uninitialised. * @filler: callback routine for filling a single page. * @data: private data for the callback routine. * * Hides the details of the LRU cache etc from the filesystems. */ int read_cache_pages(struct address_space *mapping, struct list_head *pages, int (*filler)(void *, struct page *), void *data) { struct page *page; int ret = 0; while (!list_empty(pages)) { page = list_to_page(pages); list_del(&page->lru); if (add_to_page_cache_lru(page, mapping, page->index, GFP_KERNEL)) { page_cache_release(page); continue; } page_cache_release(page); ret = filler(data, page); if (unlikely(ret)) { put_pages_list(pages); break; } task_io_account_read(PAGE_CACHE_SIZE); } return ret; } EXPORT_SYMBOL(read_cache_pages); static int read_pages(struct address_space *mapping, struct file *filp, struct list_head *pages, unsigned nr_pages) { unsigned page_idx; int ret; if (mapping->a_ops->readpages) { ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages); /* Clean up the remaining pages */ put_pages_list(pages); goto out; } for (page_idx = 0; page_idx < nr_pages; page_idx++) { struct page *page = list_to_page(pages); list_del(&page->lru); if (!add_to_page_cache_lru(page, mapping, page->index, GFP_KERNEL)) { mapping->a_ops->readpage(filp, page); } page_cache_release(page); } ret = 0; out: return ret; } /* * do_page_cache_readahead actually reads a chunk of disk. It allocates all * the pages first, then submits them all for I/O. This avoids the very bad * behaviour which would occur if page allocations are causing VM writeback. * We really don't want to intermingle reads and writes like that. * * Returns the number of pages requested, or the maximum amount of I/O allowed. * * do_page_cache_readahead() returns -1 if it encountered request queue * congestion. */ static int __do_page_cache_readahead(struct address_space *mapping, struct file *filp, pgoff_t offset, unsigned long nr_to_read, unsigned long lookahead_size) { struct inode *inode = mapping->host; struct page *page; unsigned long end_index; /* The last page we want to read */ LIST_HEAD(page_pool); int page_idx; int ret = 0; loff_t isize = i_size_read(inode); if (isize == 0) goto out; end_index = ((isize - 1) >> PAGE_CACHE_SHIFT); /* * Preallocate as many pages as we will need. */ for (page_idx = 0; page_idx < nr_to_read; page_idx++) { pgoff_t page_offset = offset + page_idx; if (page_offset > end_index) break; rcu_read_lock(); page = radix_tree_lookup(&mapping->page_tree, page_offset); rcu_read_unlock(); if (page) continue; page = page_cache_alloc_cold(mapping); if (!page) break; page->index = page_offset; list_add(&page->lru, &page_pool); if (page_idx == nr_to_read - lookahead_size) SetPageReadahead(page); ret++; } /* * Now start the IO. We ignore I/O errors - if the page is not * uptodate then the caller will launch readpage again, and * will then handle the error. */ if (ret) read_pages(mapping, filp, &page_pool, ret); BUG_ON(!list_empty(&page_pool)); out: return ret; } /* * Chunk the readahead into 2 megabyte units, so that we don't pin too much * memory at once. */ int force_page_cache_readahead(struct address_space *mapping, struct file *filp, pgoff_t offset, unsigned long nr_to_read) { int ret = 0; if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) return -EINVAL; while (nr_to_read) { int err; unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE; if (this_chunk > nr_to_read) this_chunk = nr_to_read; err = __do_page_cache_readahead(mapping, filp, offset, this_chunk, 0); if (err < 0) { ret = err; break; } ret += err; offset += this_chunk; nr_to_read -= this_chunk; } return ret; } /* * This version skips the IO if the queue is read-congested, and will tell the * block layer to abandon the readahead if request allocation would block. * * force_page_cache_readahead() will ignore queue congestion and will block on * request queues. */ int do_page_cache_readahead(struct address_space *mapping, struct file *filp, pgoff_t offset, unsigned long nr_to_read) { if (bdi_read_congested(mapping->backing_dev_info)) return -1; return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0); } /* * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a * sensible upper limit. */ unsigned long max_sane_readahead(unsigned long nr) { return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE) + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2); } static int __init readahead_init(void) { int err; err = bdi_init(&default_backing_dev_info); if (!err) bdi_register(&default_backing_dev_info, NULL, "default"); return err; } subsys_initcall(readahead_init); /* * Submit IO for the read-ahead request in file_ra_state. */ static unsigned long ra_submit(struct file_ra_state *ra, struct address_space *mapping, struct file *filp) { int actual; actual = __do_page_cache_readahead(mapping, filp, ra->start, ra->size, ra->async_size); return actual; } /* * Set the initial window size, round to next power of 2 and square * for small size, x 4 for medium, and x 2 for large * for 128k (32 page) max ra * 1-8 page = 32k initial, > 8 page = 128k initial */ static unsigned long get_init_ra_size(unsigned long size, unsigned long max) { unsigned long newsize = roundup_pow_of_two(size); if (newsize <= max / 32) newsize = newsize * 4; else if (newsize <= max / 4) newsize = newsize * 2; else newsize = max; return newsize; } /* * Get the previous window size, ramp it up, and * return it as the new window size. */ static unsigned long get_next_ra_size(struct file_ra_state *ra, unsigned long max) { unsigned long cur = ra->size; unsigned long newsize; if (cur < max / 16) newsize = 4 * cur; else newsize = 2 * cur; return min(newsize, max); } /* * On-demand readahead design. * * The fields in struct file_ra_state represent the most-recently-executed * readahead attempt: * * |<----- async_size ---------| * |------------------- size -------------------->| * |==================#===========================| * ^start ^page marked with PG_readahead * * To overlap application thinking time and disk I/O time, we do * `readahead pipelining': Do not wait until the application consumed all * readahead pages and stalled on the missing page at readahead_index; * Instead, submit an asynchronous readahead I/O as soon as there are * only async_size pages left in the readahead window. Normally async_size * will be equal to size, for maximum pipelining. * * In interleaved sequential reads, concurrent streams on the same fd can * be invalidating each other's readahead state. So we flag the new readahead * page at (start+size-async_size) with PG_readahead, and use it as readahead * indicator. The flag won't be set on already cached pages, to avoid the * readahead-for-nothing fuss, saving pointless page cache lookups. * * prev_pos tracks the last visited byte in the _previous_ read request. * It should be maintained by the caller, and will be used for detecting * small random reads. Note that the readahead algorithm checks loosely * for sequential patterns. Hence interleaved reads might be served as * sequential ones. * * There is a special-case: if the first page which the application tries to * read happens to be the first page of the file, it is assumed that a linear * read is about to happen and the window is immediately set to the initial size * based on I/O request size and the max_readahead. * * The code ramps up the readahead size aggressively at first, but slow down as * it approaches max_readhead. */ /* * A minimal readahead algorithm for trivial sequential/random reads. */ static unsigned long ondemand_readahead(struct address_space *mapping, struct file_ra_state *ra, struct file *filp, bool hit_readahead_marker, pgoff_t offset, unsigned long req_size) { int max = ra->ra_pages; /* max readahead pages */ pgoff_t prev_offset; int sequential; /* * It's the expected callback offset, assume sequential access. * Ramp up sizes, and push forward the readahead window. */ if (offset && (offset == (ra->start + ra->size - ra->async_size) || offset == (ra->start + ra->size))) { ra->start += ra->size; ra->size = get_next_ra_size(ra, max); ra->async_size = ra->size; goto readit; } prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT; sequential = offset - prev_offset <= 1UL || req_size > max; /* * Standalone, small read. * Read as is, and do not pollute the readahead state. */ if (!hit_readahead_marker && !sequential) { return __do_page_cache_readahead(mapping, filp, offset, req_size, 0); } /* * Hit a marked page without valid readahead state. * E.g. interleaved reads. * Query the pagecache for async_size, which normally equals to * readahead size. Ramp it up and use it as the new readahead size. */ if (hit_readahead_marker) { pgoff_t start; rcu_read_lock(); start = radix_tree_next_hole(&mapping->page_tree, offset,max+1); rcu_read_unlock(); if (!start || start - offset > max) return 0; ra->start = start; ra->size = start - offset; /* old async_size */ ra->size = get_next_ra_size(ra, max); ra->async_size = ra->size; goto readit; } /* * It may be one of * - first read on start of file * - sequential cache miss * - oversize random read * Start readahead for it. */ ra->start = offset; ra->size = get_init_ra_size(req_size, max); ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size; readit: return ra_submit(ra, mapping, filp); } /** * page_cache_sync_readahead - generic file readahead * @mapping: address_space which holds the pagecache and I/O vectors * @ra: file_ra_state which holds the readahead state * @filp: passed on to ->readpage() and ->readpages() * @offset: start offset into @mapping, in pagecache page-sized units * @req_size: hint: total size of the read which the caller is performing in * pagecache pages * * page_cache_sync_readahead() should be called when a cache miss happened: * it will submit the read. The readahead logic may decide to piggyback more * pages onto the read request if access patterns suggest it will improve * performance. */ void page_cache_sync_readahead(struct address_space *mapping, struct file_ra_state *ra, struct file *filp, pgoff_t offset, unsigned long req_size) { /* no read-ahead */ if (!ra->ra_pages) return; /* do read-ahead */ ondemand_readahead(mapping, ra, filp, false, offset, req_size); } EXPORT_SYMBOL_GPL(page_cache_sync_readahead); /** * page_cache_async_readahead - file readahead for marked pages * @mapping: address_space which holds the pagecache and I/O vectors * @ra: file_ra_state which holds the readahead state * @filp: passed on to ->readpage() and ->readpages() * @page: the page at @offset which has the PG_readahead flag set * @offset: start offset into @mapping, in pagecache page-sized units * @req_size: hint: total size of the read which the caller is performing in * pagecache pages * * page_cache_async_ondemand() should be called when a page is used which * has the PG_readahead flag; this is a marker to suggest that the application * has used up enough of the readahead window that we should start pulling in * more pages. */ void page_cache_async_readahead(struct address_space *mapping, struct file_ra_state *ra, struct file *filp, struct page *page, pgoff_t offset, unsigned long req_size) { /* no read-ahead */ if (!ra->ra_pages) return; /* * Same bit is used for PG_readahead and PG_reclaim. */ if (PageWriteback(page)) return; ClearPageReadahead(page); /* * Defer asynchronous read-ahead on IO congestion. */ if (bdi_read_congested(mapping->backing_dev_info)) return; /* do read-ahead */ ondemand_readahead(mapping, ra, filp, true, offset, req_size); } EXPORT_SYMBOL_GPL(page_cache_async_readahead);