linux-kernel - Re: [PATCH mm-unstable v2] mm/page_alloc: keep track of free highatomic

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Message-ID: <CAOUHufaS-dGAPGs1Y1=imW_nusaTDeysN3qfJc9-76DBVEHScQ@mail.gmail.com>
Date: Sun, 27 Oct 2024 14:17:33 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Zi Yan <ziy@...dia.com>, Mel Gorman <mgorman@...hsingularity.net>, 
	Matt Fleming <mfleming@...udflare.com>, David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Link Lin <linkl@...gle.com>
Subject: Re: [PATCH mm-unstable v2] mm/page_alloc: keep track of free highatomic

On Sun, Oct 27, 2024 at 1:53 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 10/26/24 05:36, Yu Zhao wrote:
> > OOM kills due to vastly overestimated free highatomic reserves were
> > observed:
> >
> >   ... invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0 ...
> >   Node 0 Normal free:1482936kB boost:0kB min:410416kB low:739404kB high:1068392kB reserved_highatomic:1073152KB ...
> >   Node 0 Normal: 1292*4kB (ME) 1920*8kB (E) 383*16kB (UE) 220*32kB (ME) 340*64kB (E) 2155*128kB (UE) 3243*256kB (UE) 615*512kB (U) 1*1024kB (M) 0*2048kB 0*4096kB = 1477408kB
> >
> > The second line above shows that the OOM kill was due to the following
> > condition:
> >
> >   free (1482936kB) - reserved_highatomic (1073152kB) = 409784KB < min (410416kB)
> >
> > And the third line shows there were no free pages in any
> > MIGRATE_HIGHATOMIC pageblocks, which otherwise would show up as type
> > 'H'. Therefore __zone_watermark_unusable_free() underestimated the
> > usable free memory by over 1GB, which resulted in the unnecessary OOM
> > kill above.
> >
> > The comments in __zone_watermark_unusable_free() warns about the
> > potential risk, i.e.,
> >
> >   If the caller does not have rights to reserves below the min
> >   watermark then subtract the high-atomic reserves. This will
> >   over-estimate the size of the atomic reserve but it avoids a search.
> >
> > However, it is possible to keep track of free pages in reserved
> > highatomic pageblocks with a new per-zone counter nr_free_highatomic
> > protected by the zone lock, to avoid a search when calculating the
>
> It's only possible to track this reliably since the "mm: page_alloc:
> freelist migratetype hygiene" patchset was merged, which explains why
> nr_reserved_highatomic was used until now, even if it's imprecise.

I just refreshed my memory by quickly going through the discussion
around that series and didn't find anything that helps me understand
the above. More pointers please?