Re: [PATCH 1/3] mm: page allocator: Update free page counters after pages are placed on the free list

[Rik van Riel <riel@redhat.com>]

On 08/16/2010 05:42 AM, Mel Gorman wrote:
> When allocating a page, the system uses NR_FREE_PAGES counters to determine
> if watermarks would remain intact after the allocation was made. This
> check is made without interrupts disabled or the zone lock held and so is
> race-prone by nature. Unfortunately, when pages are being freed in batch,
> the counters are updated before the pages are added on the list. During this
> window, the counters are misleading as the pages do not exist yet. When
> under significant pressure on systems with large numbers of CPUs, it's
> possible for processes to make progress even though they should have been
> stalled. This is particularly problematic if a number of the processes are
> using GFP_ATOMIC as the min watermark can be accidentally breached and in
> extreme cases, the system can livelock.
>
> This patch updates the counters after the pages have been added to the
> list. This makes the allocator more cautious with respect to preserving
> the watermarks and mitigates livelock possibilities.
>
> Signed-off-by: Mel Gorman<mel@csn.ul.ie>

Reviewed-by: Rik van Riel <riel@redhat.com>

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