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

[KOSAKI Motohiro]

> 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@xxxxxxxxx>
> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
> Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>




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