Commit c8b74c2f authored by Sonny Rao's avatar Sonny Rao Committed by Linus Torvalds
Browse files

mm: fix calculation of dirtyable memory 



The system uses global_dirtyable_memory() to calculate number of
dirtyable pages/pages that can be allocated to the page cache.  A bug
causes an underflow thus making the page count look like a big unsigned
number.  This in turn confuses the dirty writeback throttling to
aggressively write back pages as they become dirty (usually 1 page at a
time).  This generally only affects systems with highmem because the
underflowed count gets subtracted from the global count of dirtyable
memory.

The problem was introduced with v3.2-4896-gab8fabd

Fix is to ensure we don't get an underflowed total of either highmem or
global dirtyable memory.

Signed-off-by: default avatarSonny Rao <sonnyrao@chromium.org>
Signed-off-by: default avatarPuneet Kumar <puneetster@chromium.org>
Acked-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Tested-by: default avatarDamien Wyart <damien.wyart@free.fr>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 010fc29a

mm/page-writeback.c

+20 −5
Original line number Diff line number Diff line
@@ -200,6 +200,18 @@ static unsigned long highmem_dirtyable_memory(unsigned long total) 
		x += zone_page_state(z, NR_FREE_PAGES) +

		     zone_reclaimable_pages(z) - z->dirty_balance_reserve;

	}

	/*

	 * Unreclaimable memory (kernel memory or anonymous memory

	 * without swap) can bring down the dirtyable pages below

	 * the zone's dirty balance reserve and the above calculation

	 * will underflow.  However we still want to add in nodes

	 * which are below threshold (negative values) to get a more

	 * accurate calculation but make sure that the total never

	 * underflows.

	 */

	if ((long)x < 0)

		x = 0;



	/*

	 * Make sure that the number of highmem pages is never larger

	 * than the number of the total dirtyable memory. This can only
@@ -222,8 +234,8 @@ static unsigned long global_dirtyable_memory(void) 
{

	unsigned long x;



	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages() -

	    dirty_balance_reserve;

	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();

	x -= min(x, dirty_balance_reserve);



	if (!vm_highmem_is_dirtyable)

		x -= highmem_dirtyable_memory(x);
@@ -290,9 +302,12 @@ static unsigned long zone_dirtyable_memory(struct zone *zone) 
	 * highmem zone can hold its share of dirty pages, so we don't

	 * care about vm_highmem_is_dirtyable here.

	 */

	return zone_page_state(zone, NR_FREE_PAGES) +

	       zone_reclaimable_pages(zone) -

	       zone->dirty_balance_reserve;

	unsigned long nr_pages = zone_page_state(zone, NR_FREE_PAGES) +

		zone_reclaimable_pages(zone);



	/* don't allow this to underflow */

	nr_pages -= min(nr_pages, zone->dirty_balance_reserve);

	return nr_pages;

}



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