path: root/mm/page-writeback.c

/*
 * 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