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

[Yu Zhao <yuzhao@google.com>]

On Fri, Oct 25, 2024 at 10:24 PM Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> On Fri, 25 Oct 2024 21:36:25 -0600 Yu Zhao <yuzhao@google.com> 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
>
> Under what circumstances?

Link wrote a repro, so he'd be the best person to answer this question.

Link, please help. Thanks.

> > 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
> > usable free memory. And the cost would be minimal, i.e., simple
> > arithmetics in the highatomic alloc/free/move paths.
>
> Is a -stable backport needed?
>
> If so, is a Fixes: target identifiable?

The code has been there for many years, and we only recently noticed
the problem from Link's repro. So it doesn't look like a stable
material.