Message-ID: <CAOUHufanF3VaLzq6o_V+-+iPvB4Oj-xHwD+Rm-gmKS02h8Dw=g@mail.gmail.com>
Date: Wed, 23 Oct 2024 00:36:46 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Michal Hocko <mhocko@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Link Lin <linkl@...gle.com>, Mel Gorman <mgorman@...hsingularity.net>, 
	Matt Fleming <mfleming@...udflare.com>
Subject: Re: [PATCH mm-unstable v1] mm/page_alloc: try not to overestimate
 free highatomic

On Tue, Oct 22, 2024 at 4:53 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> +Cc Mel and Matt
>
> On 10/21/24 19:25, Michal Hocko wrote:
> > On Mon 21-10-24 11:10:50, Yu Zhao wrote:
> >> On Mon, Oct 21, 2024 at 2:13 AM Michal Hocko <mhocko@...e.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
>
> Hm I don't think it's completely WAI. The intention is that we should be
> able to unreserve the highatomic pageblocks before going OOM, and there
> seems to be an unintended corner case that if the pageblocks are fully
> exhausted, they are not reachable for unreserving.

I still think unreserving should only apply to highatomic PBs that
contain free pages. Otherwise, it seems to me that it'd be
self-defecting because:
1. Unreserving fully used hightatomic PBs can't fulfill the alloc
demand immediately.
2. More importantly, it only takes one alloc failure in
__alloc_pages_direct_reclaim() to reset nr_reserved_highatomic to 2MB,
from as high as 1% of a zone (in this case 1GB). IOW, it makes more
sense to me that highatomic only unreserves what it doesn't fully use
each time unreserve_highatomic_pageblock() is called, not everything
it got (except the last PB).

Also not reachable from free_area[] isn't really a big problem. There
are ways to solve this without scanning the PB bitmap.

> The nr_highatomic is then
> also fully misleading as it prevents allocations due to a limit that does
> not reflect reality.

Right, and the comments warn about this.

> Your patch addresses the second issue, but there's a
> cost to it when calculating the watermarks, and it would be better to
> address the root issue instead.

Theoretically, yes. And I don't think it's actually measurable
considering the paths (alloc/reclaim) we are in -- all the data
structures this patch accesses should already have been cache-hot, due
to unreserve_highatomic_pageblock(), etc.

Also, we have not agreed on the root cause yet.

> >> that those pages can become usable to others. There is nothing to
> >> unreserve when they have no free pages.
>
> Yeah there are no actual free pages to unreserve, but unreserving would fix
> the nr_highatomic overestimate and thus allow allocations to proceed.

Yes, but honestly, I think this is going to cause regression in
highatomic allocs.

> > I do not follow. How can you have reserved highatomic pages of that size
> > without having page blocks with free memory. In other words is this an
> > accounting problem or reserves problem? This is not really clear from
> > your description.
>
> I think it's the problem of finding the highatomic pageblocks for
> unreserving them once they become full. The proper fix is not exactly
> trivial though. Either we'll have to scan for highatomic pageblocks in the
> pageblock bitmap, or track them using an additional data structure.

Assuming we want to unreserve fully used hightatomic PBs, we wouldn't
need to scan for them or track them. We'd only need to track the delta
between how many we want to unreserve (full or not) and how many we
are able to do so. The first page freed in a PB that's highatomic
would need to try to reduce the delta by changing the MT.

To summarize, I think this is an estimation problem, which I would
categorize as a lesser problem than accounting problems. But it sounds
to me that you think it's a policy problem, i.e., the highatomic
unreserving policy is wrong or not properly implemented?

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