Re: [PATCH mm-unstable v1] mm/page_alloc: try not to overestimate free highatomic

[Yu Zhao <yuzhao@google.com>]

On Mon, Oct 21, 2024 at 11:26 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 21-10-24 11:10:50, Yu Zhao wrote:
> > On Mon, Oct 21, 2024 at 2:13 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Sat 19-10-24 23:13:15, 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() overestimated free
> > > > highatomic reserves. IOW, it underestimated the usable free memory by
> > > > over 1GB, which resulted in the unnecessary OOM kill.
> > >
> > > Why doesn't unreserve_highatomic_pageblock deal with this situation?
> >
> > The current behavior of unreserve_highatomic_pageblock() seems WAI to
> > me: it unreserves highatomic pageblocks that contain *free* pages so
> > that those pages can become usable to others. There is nothing to
> > unreserve when they have no free pages.
>
> I do not follow. How can you have reserved highatomic pages of that size
> without having page blocks with free memory.

Sorry I might still not get your question: are you saying it's not
possible for 524 pageblocks (reserved_highatomic=1073152kB) not to
have free pages? It might be uncommon but I don't think it's
impossible.

> In other words is this an
> accounting problem or reserves problem?

I don't follow here: why does it need to be one of the two?
reserved_highatomic can go up to 1% of the zone, and all reserves can
be used for highatomic allocs, leaving no free pages in
reserved_highatomic.