/* * mm/page-writeback.c. * * Copyright (C) 2002, Linus Torvalds. * * Contains functions related to writing back dirty pages at the * address_space level. * * 10Apr2002 akpm@zip.com.au * Initial version */ #include <linux/kernel.h> #include <linux/module.h> #include <linux/spinlock.h> #include <linux/fs.h> #include <linux/mm.h> #include <linux/swap.h> #include <linux/slab.h> #include <linux/pagemap.h> #include <linux/writeback.h> #include <linux/init.h> #include <linux/backing-dev.h> #include <linux/blkdev.h> #include <linux/mpage.h> #include <linux/percpu.h> #include <linux/notifier.h> #include <linux/smp.h> #include <linux/sysctl.h> #include <linux/cpu.h> #include <linux/syscalls.h> /* * The maximum number of pages to writeout in a single bdflush/kupdate * operation. We do this so we don't hold I_LOCK against an inode for * enormous amounts of time, which would block a userspace task which has * been forced to throttle against that inode. Also, the code reevaluates * the dirty each time it has written this many pages. */ #define MAX_WRITEBACK_PAGES 1024 /* * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited * will look to see if it needs to force writeback or throttling. */ static long ratelimit_pages = 32; static long total_pages; /* The total number of pages in the machine. */ static int dirty_exceeded __cacheline_aligned_in_smp; /* Dirty mem may be over limit */ /* * When balance_dirty_pages decides that the caller needs to perform some * non-background writeback, this is how many pages it will attempt to write. * It should be somewhat larger than RATELIMIT_PAGES to ensure that reasonably * large amounts of I/O are submitted. */ static inline long sync_writeback_pages(void) { return ratelimit_pages + ratelimit_pages / 2; } /* The following parameters are exported via /proc/sys/vm */ /* * Start background writeback (via pdflush) at this percentage */ int dirty_background_ratio = 10; /* * The generator of dirty data starts writeback at this percentage */ int vm_dirty_ratio = 40; /* * The interval between `kupdate'-style writebacks, in centiseconds * (hundredths of a second) */ int dirty_writeback_centisecs = 5 * 100; /* * The longest number of centiseconds for which data is allowed to remain dirty */ int dirty_expire_centisecs = 30 * 100; /* * Flag that makes the machine dump writes/reads and block dirtyings. */ int block_dump; /* * Flag that puts the machine in "laptop mode". */ int laptop_mode; EXPORT_SYMBOL(laptop_mode); /* End of sysctl-exported parameters */ static void background_writeout(unsigned long _min_pages); struct writeback_state { unsigned long nr_dirty; unsigned long nr_unstable; unsigned long nr_mapped; unsigned long nr_writeback; }; static void get_writeback_state(struct writeback_state *wbs) { wbs->nr_dirty = read_page_state(nr_dirty); wbs->nr_unstable = read_page_state(nr_unstable); wbs->nr_mapped = read_page_state(nr_mapped); wbs->nr_writeback = read_page_state(nr_writeback); } /* * Work out the current dirty-memory clamping and background writeout * thresholds. * * The main aim here is to lower them aggressively if there is a lot of mapped * memory around. To avoid stressing page reclaim with lots of unreclaimable * pages. It is better to clamp down on writers than to start swapping, and * performing lots of scanning. * * We only allow 1/2 of the currently-unmapped memory to be dirtied. * * We don't permit the clamping level to fall below 5% - that is getting rather * excessive. * * We make sure that the background writeout level is below the adjusted * clamping level. */ static void get_dirty_limits(struct writeback_state *wbs, long *pbackground, long *pdirty, struct address_space *mapping) { int background_ratio; /* Percentages */ int dirty_ratio; int unmapped_ratio; long background; long dirty; unsigned long available_memory = total_pages; struct task_struct *tsk; get_writeback_state(wbs); #ifdef CONFIG_HIGHMEM /* * If this mapping can only allocate from low memory, * we exclude high memory from our count. */ if (mapping && !(mapping_gfp_mask(mapping) & __GFP_HIGHMEM)) available_memory -= totalhigh_pages; #endif unmapped_ratio = 100 - (wbs->nr_mapped * 100) / total_pages; dirty_ratio = vm_dirty_ratio; if (dirty_ratio > unmapped_ratio / 2) dirty_ratio = unmapped_ratio / 2; if (dirty_ratio < 5) dirty_ratio = 5; background_ratio = dirty_background_ratio; if (background_ratio >= dirty_ratio) background_ratio = dirty_ratio / 2; background = (background_ratio * available_memory) / 100; dirty = (dirty_ratio * available_memory) / 100; tsk = current; if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { background += background / 4; dirty += dirty / 4; } *pbackground = background; *pdirty = dirty; } /* * balance_dirty_pages() must be called by processes which are generating dirty * data. It looks at the number of dirty pages in the machine and will force * the caller to perform writeback if the system is over `vm_dirty_ratio'. * If we're over `background_thresh' then pdflush is woken to perform some * writeout. */ static void balance_dirty_pages(struct address_space *mapping) { struct writeback_state wbs; long nr_reclaimable; long background_thresh; long dirty_thresh; unsigned long pages_written = 0; unsigned long write_chunk = sync_writeback_pages(); struct backing_dev_info *bdi = mapping->backing_dev_info; for (;;) { struct writeback_control wbc = { .bdi = bdi, .sync_mode = WB_SYNC_NONE, .older_than_this = NULL, .nr_to_write = write_chunk, }; get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, mapping); nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable; if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh) break; if (!dirty_exceeded) dirty_exceeded = 1; /* Note: nr_reclaimable denotes nr_dirty + nr_unstable. * Unstable writes are a feature of certain networked * filesystems (i.e. NFS) in which data may have been * written to the server's write cache, but has not yet * been flushed to permanent storage. */ if (nr_reclaimable) { writeback_inodes(&wbc); get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, mapping); nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable; if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh) break; pages_written += write_chunk - wbc.nr_to_write; if (pages_written >= write_chunk) break; /* We've done our duty */ } blk_congestion_wait(WRITE, HZ/10); } if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh && dirty_exceeded) dirty_exceeded = 0; if (writeback_in_progress(bdi)) return; /* pdflush is already working this queue */ /* * In laptop mode, we wait until hitting the higher threshold before * starting background writeout, and then write out all the way down * to the lower threshold. So slow writers cause minimal disk activity. * * In normal mode, we start background writeout at the lower * background_thresh, to keep the amount of dirty memory low. */ if ((laptop_mode && pages_written) || (!laptop_mode && (nr_reclaimable > background_thresh))) pdflush_operation(background_writeout, 0); } /** * balance_dirty_pages_ratelimited - balance dirty memory state * @mapping: address_space which was dirtied * * Processes which are dirtying memory should call in here once for each page * which was newly dirtied. The function will periodically check the system's * dirty state and will initiate writeback if needed. * * On really big machines, get_writeback_state is expensive, so try to avoid * calling it too often (ratelimiting). But once we're over the dirty memory * limit we decrease the ratelimiting by a lot, to prevent individual processes * from overshooting the limit by (ratelimit_pages) each. */ void balance_dirty_pages_ratelimited(struct address_space *mapping) { static DEFINE_PER_CPU(int, ratelimits) = 0; long ratelimit; ratelimit = ratelimit_pages; if (dirty_exceeded) ratelimit = 8; /* * Check the rate limiting. Also, we do not want to throttle real-time * tasks in balance_dirty_pages(). Period. */ if (get_cpu_var(ratelimits)++ >= ratelimit) { __get_cpu_var(ratelimits) = 0; put_cpu_var(ratelimits); balance_dirty_pages(mapping); return; } put_cpu_var(ratelimits); } EXPORT_SYMBOL(balance_dirty_pages_ratelimited); void throttle_vm_writeout(void) { struct writeback_state wbs; long background_thresh; long dirty_thresh; for ( ; ; ) { get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, NULL); /* * Boost the allowable dirty threshold a bit for page * allocators so they don't get DoS'ed by heavy writers */ dirty_thresh += dirty_thresh / 10; /* wheeee... */ if (wbs.nr_unstable + wbs.nr_writeback <= dirty_thresh) break; blk_congestion_wait(WRITE, HZ/10); } } /* * writeback at least _min_pages, and keep writing until the amount of dirty * memory is less than the background threshold, or until we're all clean. */ static void background_writeout(unsigned long _min_pages) { long min_pages = _min_pages; struct writeback_control wbc = { .bdi = NULL, .sync_mode = WB_SYNC_NONE, .older_than_this = NULL, .nr_to_write = 0, .nonblocking = 1, }; for ( ; ; ) { struct writeback_state wbs; long background_thresh; long dirty_thresh; get_dirty_limits(&wbs, &background_thresh, &dirty_thresh, NULL); if (wbs.nr_dirty + wbs.nr_unstable < background_thresh && min_pages <= 0) break; wbc.encountered_congestion = 0; wbc.nr_to_write = MAX_WRITEBACK_PAGES; wbc.pages_skipped = 0; writeback_inodes(&wbc); min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write; if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) { /* Wrote less than expected */ blk_congestion_wait(WRITE, HZ/10); if (!wbc.encountered_congestion) break; } } } /* * Start writeback of `nr_pages' pages. If `nr_pages' is zero, write back * the whole world. Returns 0 if a pdflush thread was dispatched. Returns * -1 if all pdflush threads were busy. */ int wakeup_pdflush(long nr_pages) { if (nr_pages == 0) { struct writeback_state wbs; get_writeback_state(&wbs); nr_pages = wbs.nr_dirty + wbs.nr_unstable; } return pdflush_operation(background_writeout, nr_pages); } static void wb_timer_fn(unsigned long unused); static void laptop_timer_fn(unsigned long unused); static DEFINE_TIMER(wb_timer, wb_timer_fn, 0, 0); static DEFINE_TIMER(laptop_mode_wb_timer, laptop_timer_fn, 0, 0); /* * Periodic writeback of "old" data. * * Define "old": the first time one of an inode's pages is dirtied, we mark the * dirtying-time in the inode's address_space. So this periodic writeback code * just walks the superblock inode list, writing back any inodes which are * older than a specific point in time. * * Try to run once per dirty_writeback_centisecs. But if a writeback event * takes longer than a dirty_writeback_centisecs interval, then leave a * one-second gap. * * older_than_this takes precedence over nr_to_write. So we'll only write back * all dirty pages if they are all attached to "old" mappings. */ static void wb_kupdate(unsigned long arg) { unsigned long oldest_jif; unsigned long start_jif; unsigned long next_jif; long nr_to_write; struct writeback_state wbs; struct writeback_control wbc = { .bdi = NULL, .sync_mode = WB_SYNC_NONE, .older_than_this = &oldest_jif, .nr_to_write = 0, .nonblocking = 1, .for_kupdate = 1, }; sync_supers(); get_writeback_state(&wbs); oldest_jif = jiffies - (dirty_expire_centisecs * HZ) / 100; start_jif = jiffies; next_jif = start_jif + (dirty_writeback_centisecs * HZ) / 100; nr_to_write = wbs.nr_dirty + wbs.nr_unstable + (inodes_stat.nr_inodes - inodes_stat.nr_unused); while (nr_to_write > 0) { wbc.encountered_congestion = 0; wbc.nr_to_write = MAX_WRITEBACK_PAGES; writeback_inodes(&wbc); if (wbc.nr_to_write > 0) { if (wbc.encountered_congestion) blk_congestion_wait(WRITE, HZ/10); else break; /* All the old data is written */ } nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write; } if (time_before(next_jif, jiffies + HZ)) next_jif = jiffies + HZ; if (dirty_writeback_centisecs) mod_timer(&wb_timer, next_jif); } /* * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs */ int dirty_writeback_centisecs_handler(ctl_table *table, int write, struct file *file, void __user *buffer, size_t *length, loff_t *ppos) { proc_dointvec(table, write, file, buffer, length, ppos); if (dirty_writeback_centisecs) { mod_timer(&wb_timer, jiffies + (dirty_writeback_centisecs * HZ) / 100); } else { del_timer(&wb_timer); } return 0; } static void wb_timer_fn(unsigned long unused) { if (pdflush_operation(wb_kupdate, 0) < 0) mod_timer(&wb_timer, jiffies + HZ); /* delay 1 second */ } static void laptop_flush(unsigned long unused) { sys_sync(); } static void laptop_timer_fn(unsigned long unused) { pdflush_operation(laptop_flush, 0); } /* * We've spun up the disk and we're in laptop mode: schedule writeback * of all dirty data a few seconds from now. If the flush is already scheduled * then push it back - the user is still using the disk. */ void laptop_io_completion(void) { mod_timer(&laptop_mode_wb_timer, jiffies + laptop_mode * HZ); } /* * We're in laptop mode and we've just synced. The sync's writes will have * caused another writeback to be scheduled by laptop_io_completion. * Nothing needs to be written back anymore, so we unschedule the writeback. */ void laptop_sync_completion(void) { del_timer(&laptop_mode_wb_timer); } /* * If ratelimit_pages is too high then we can get into dirty-data overload * if a large number of processes all perform writes at the same time. * If it is too low then SMP machines will call the (expensive) * get_writeback_state too often. * * Here we set ratelimit_pages to a level which ensures that when all CPUs are * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory * thresholds before writeback cuts in. * * But the limit should not be set too high. Because it also controls the * amount of memory which the balance_dirty_pages() caller has to write back. * If this is too large then the caller will block on the IO queue all the * time. So limit it to four megabytes - the balance_dirty_pages() caller * will write six megabyte chunks, max. */ static void set_ratelimit(void) { ratelimit_pages = total_pages / (num_online_cpus() * 32); if (ratelimit_pages < 16) ratelimit_pages = 16; if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024) ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE; } static int ratelimit_handler(struct notifier_block *self, unsigned long u, void *v) { set_ratelimit(); return 0; } static struct notifier_block ratelimit_nb = { .notifier_call = ratelimit_handler, .next = NULL, }; /* * If the machine has a large highmem:lowmem ratio then scale back the default * dirty memory thresholds: allowing too much dirty highmem pins an excessive * number of buffer_heads. */ void __init page_writeback_init(void) { long buffer_pages = nr_free_buffer_pages(); long correction; total_pages = nr_free_pagecache_pages(); correction = (100 * 4 * buffer_pages) / total_pages; if (correction < 100) { dirty_background_ratio *= correction; dirty_background_ratio /= 100; vm_dirty_ratio *= correction; vm_dirty_ratio /= 100; if (dirty_background_ratio <= 0) dirty_background_ratio = 1; if (vm_dirty_ratio <= 0) vm_dirty_ratio = 1; } mod_timer(&wb_timer, jiffies + (dirty_writeback_centisecs * HZ) / 100); set_ratelimit(); register_cpu_notifier(&ratelimit_nb); } int do_writepages(struct address_space *mapping, struct writeback_control *wbc) { int ret; if (wbc->nr_to_write <= 0) return 0; wbc->for_writepages = 1; if (mapping->a_ops->writepages) ret = mapping->a_ops->writepages(mapping, wbc); else ret = generic_writepages(mapping, wbc); wbc->for_writepages = 0; return ret; } /** * write_one_page - write out a single page and optionally wait on I/O * * @page: the page to write * @wait: if true, wait on writeout * * The page must be locked by the caller and will be unlocked upon return. * * write_one_page() returns a negative error code if I/O failed. */ int write_one_page(struct page *page, int wait) { struct address_space *mapping = page->mapping; int ret = 0; struct writeback_control wbc = { .sync_mode = WB_SYNC_ALL, .nr_to_write = 1, }; BUG_ON(!PageLocked(page)); if (wait) wait_on_page_writeback(page); if (clear_page_dirty_for_io(page)) { page_cache_get(page); ret = mapping->a_ops->writepage(page, &wbc); if (ret == 0 && wait) { wait_on_page_writeback(page); if (PageError(page)) ret = -EIO; } page_cache_release(page); } else { unlock_page(page); } return ret; } EXPORT_SYMBOL(write_one_page); /* * For address_spaces which do not use buffers. Just tag the page as dirty in * its radix tree. * * This is also used when a single buffer is being dirtied: we want to set the * page dirty in that case, but not all the buffers. This is a "bottom-up" * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying. * * Most callers have locked the page, which pins the address_space in memory. * But zap_pte_range() does not lock the page, however in that case the * mapping is pinned by the vma's ->vm_file reference. * * We take care to handle the case where the page was truncated from the * mapping by re-checking page_mapping() insode tree_lock. */ int __set_page_dirty_nobuffers(struct page *page) { int ret = 0; if (!TestSetPageDirty(page)) { struct address_space *mapping = page_mapping(page); struct address_space *mapping2; if (mapping) { write_lock_irq(&mapping->tree_lock); mapping2 = page_mapping(page); if (mapping2) { /* Race with truncate? */ BUG_ON(mapping2 != mapping); if (mapping_cap_account_dirty(mapping)) inc_page_state(nr_dirty); radix_tree_tag_set(&mapping->page_tree, page_index(page), PAGECACHE_TAG_DIRTY); } write_unlock_irq(&mapping->tree_lock); if (mapping->host) { /* !PageAnon && !swapper_space */ __mark_inode_dirty(mapping->host, I_DIRTY_PAGES); } } } return ret; } EXPORT_SYMBOL(__set_page_dirty_nobuffers); /* * When a writepage implementation decides that it doesn't want to write this * page for some reason, it should redirty the locked page via * redirty_page_for_writepage() and it should then unlock the page and return 0 */ int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page) { wbc->pages_skipped++; return __set_page_dirty_nobuffers(page); } EXPORT_SYMBOL(redirty_page_for_writepage); /* * If the mapping doesn't provide a set_page_dirty a_op, then * just fall through and assume that it wants buffer_heads. */ int fastcall set_page_dirty(struct page *page) { struct address_space *mapping = page_mapping(page); if (likely(mapping)) { int (*spd)(struct page *) = mapping->a_ops->set_page_dirty; if (spd) return (*spd)(page); return __set_page_dirty_buffers(page); } if (!PageDirty(page)) SetPageDirty(page); return 0; } EXPORT_SYMBOL(set_page_dirty); /* * set_page_dirty() is racy if the caller has no reference against * page->mapping->host, and if the page is unlocked. This is because another * CPU could truncate the page off the mapping and then free the mapping. * * Usually, the page _is_ locked, or the caller is a user-space process which * holds a reference on the inode by having an open file. * * In other cases, the page should be locked before running set_page_dirty(). */ int set_page_dirty_lock(struct page *page) { int ret; lock_page(page); ret = set_page_dirty(page); unlock_page(page); return ret; } EXPORT_SYMBOL(set_page_dirty_lock); /* * Clear a page's dirty flag, while caring for dirty memory accounting. * Returns true if the page was previously dirty. */ int test_clear_page_dirty(struct page *page) { struct address_space *mapping = page_mapping(page); unsigned long flags; if (mapping) { write_lock_irqsave(&mapping->tree_lock, flags); if (TestClearPageDirty(page)) { radix_tree_tag_clear(&mapping->page_tree, page_index(page), PAGECACHE_TAG_DIRTY); write_unlock_irqrestore(&mapping->tree_lock, flags); if (mapping_cap_account_dirty(mapping)) dec_page_state(nr_dirty); return 1; } write_unlock_irqrestore(&mapping->tree_lock, flags); return 0; } return TestClearPageDirty(page); } EXPORT_SYMBOL(test_clear_page_dirty); /* * Clear a page's dirty flag, while caring for dirty memory accounting. * Returns true if the page was previously dirty. * * This is for preparing to put the page under writeout. We leave the page * tagged as dirty in the radix tree so that a concurrent write-for-sync * can discover it via a PAGECACHE_TAG_DIRTY walk. The ->writepage * implementation will run either set_page_writeback() or set_page_dirty(), * at which stage we bring the page's dirty flag and radix-tree dirty tag * back into sync. * * This incoherency between the page's dirty flag and radix-tree tag is * unfortunate, but it only exists while the page is locked. */ int clear_page_dirty_for_io(struct page *page) { struct address_space *mapping = page_mapping(page); if (mapping) { if (TestClearPageDirty(page)) { if (mapping_cap_account_dirty(mapping)) dec_page_state(nr_dirty); return 1; } return 0; } return TestClearPageDirty(page); } EXPORT_SYMBOL(clear_page_dirty_for_io); int test_clear_page_writeback(struct page *page) { struct address_space *mapping = page_mapping(page); int ret; if (mapping) { unsigned long flags; write_lock_irqsave(&mapping->tree_lock, flags); ret = TestClearPageWriteback(page); if (ret) radix_tree_tag_clear(&mapping->page_tree, page_index(page), PAGECACHE_TAG_WRITEBACK); write_unlock_irqrestore(&mapping->tree_lock, flags); } else { ret = TestClearPageWriteback(page); } return ret; } int test_set_page_writeback(struct page *page) { struct address_space *mapping = page_mapping(page); int ret; if (mapping) { unsigned long flags; write_lock_irqsave(&mapping->tree_lock, flags); ret = TestSetPageWriteback(page); if (!ret) radix_tree_tag_set(&mapping->page_tree, page_index(page), PAGECACHE_TAG_WRITEBACK); if (!PageDirty(page)) radix_tree_tag_clear(&mapping->page_tree, page_index(page), PAGECACHE_TAG_DIRTY); write_unlock_irqrestore(&mapping->tree_lock, flags); } else { ret = TestSetPageWriteback(page); } return ret; } EXPORT_SYMBOL(test_set_page_writeback); /* * Return true if any of the pages in the mapping are marged with the * passed tag. */ int mapping_tagged(struct address_space *mapping, int tag) { unsigned long flags; int ret; read_lock_irqsave(&mapping->tree_lock, flags); ret = radix_tree_tagged(&mapping->page_tree, tag); read_unlock_irqrestore(&mapping->tree_lock, flags); return ret; } EXPORT_SYMBOL(mapping_tagged);