Re: [PATCH] vmscan: fix zone balance check in prepare_kswapd_sleep

[Vlastimil Babka <vbabka@suse.cz>]

On 02/06/2017 05:17 PM, Mel Gorman wrote:

>>
>> Thanks,
>> Shantanu
>
>> From 46f2e4b02ac263bf50d69cdab3bcbd7bcdea7415 Mon Sep 17 00:00:00 2001
>> From: Shantanu Goel <sgoel01@yahoo.com>
>> Date: Sat, 4 Feb 2017 19:07:53 -0500
>> Subject: [PATCH] vmscan: fix zone balance check in prepare_kswapd_sleep
>>
>> The check in prepare_kswapd_sleep needs to match the one in balance_pgdat
>> since the latter will return as soon as any one of the zones in the
>> classzone is above the watermark.  This is specially important for
>> higher order allocations since balance_pgdat will typically reset
>> the order to zero relying on compaction to create the higher order
>> pages.  Without this patch, prepare_kswapd_sleep fails to wake up
>> kcompactd since the zone balance check fails.
>>
>> Signed-off-by: Shantanu Goel <sgoel01@yahoo.com>
>
> I don't recall specifically why I made that change but I've no objections
> to the patch so;
>
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
>
> However, note that there is a slight risk that kswapd will sleep for a
> short interval early due to a very small zone such as ZONE_DMA. If this
> is a general problem then it'll manifest as less kswapd reclaim and more
> direct reclaim. If it turns out this is an issue then a revert will not
> be the right fix. Instead, all the checks for zone_balance will need to
> account for the only balanced zone being a tiny percentage of memory in
> the node.

Hopefully the lowmem reserves should take care of this in that case? They easily 
make a low zone inaccessible even when fully free. Unless the small zone is high 
one though, such as Normal zone on system with only 4GB memory, so most of it is 
in DMA32.

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