Re: [PATCH] mm/page_alloc.c: Avoid infinite retries caused by cpuset race

[Vlastimil Babka <vbabka@suse.cz>]

On 4/23/25 17:35, Suren Baghdasaryan wrote:
>> >> There's a new 'MEMORY MANAGEMENT - PAGE ALLOCATOR' entry (only in
>> >> Andrew's mm.git repository now).
>> >>
>> >> Let's Cc the page allocator folks here!
>> >>
>> >> --
>> >> Cheers,
>> >> Harry / Hyeonggon
>> >>
>> >>>   mm/page_alloc.c | 8 ++++++++
>> >>>   1 file changed, 8 insertions(+)
>> >>>
>> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> >>> index fd6b865cb1ab..1e82f5214a42 100644
>> >>> --- a/mm/page_alloc.c
>> >>> +++ b/mm/page_alloc.c
>> >>> @@ -4530,6 +4530,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>> >>>        }
>> >>>
>> >>>   retry:
>> >>> +     /*
>> >>> +      * Deal with possible cpuset update races or zonelist updates to avoid
>> >>> +      * infinite retries.
>> >>> +      */
>> >>> +     if (check_retry_cpuset(cpuset_mems_cookie, ac) ||
>> >>> +         check_retry_zonelist(zonelist_iter_cookie))
>> >>> +             goto restart;
>> >>> +
>> > We have this check later in this block:
>> > https://elixir.bootlin.com/linux/v6.15-rc3/source/mm/page_alloc.c#L4652,
>> > so IIUC you effectively are moving it to be called before
>> > should_reclaim_retry(). If so, I think you should remove the old one
>> > (the one I linked earlier) as it seems to be unnecessary duplication
>> > at this point.
>> In my understanding, the code in
>>
>> https://elixir.bootlin.com/linux/v6.15-rc3/source/mm/page_alloc.c#L4652
>>
>> was introduced to prevent unnecessary OOM (Out-of-Memory) conditions
>> in__alloc_pages_may_oom.
>>
>> If old code is removed, the newly added code (on retry loop entry)
>> cannot guarantee that the cpuset
>>
>> remains valid when the flow reaches in__alloc_pages_may_oom, especially
>> if scheduling occurs during this section.
> 
> Well, rescheduling can happen even between
> https://elixir.bootlin.com/linux/v6.15-rc3/source/mm/page_alloc.c#L4652
> and https://elixir.bootlin.com/linux/v6.15-rc3/source/mm/page_alloc.c#L4657
> but I see your point. Also should_reclaim_retry() does not include

I think the rescheduling isn't a problem because what we're testing is "we
are about to oom, could it have been because we raced?" and the race would
have affected the code before #L4652. If we didn't race and yet determined
it's time for oom, a race between #L4652 and #L4657 shouldn't matter. The
get_page_from_freelist() in __alloc_pages_may_oom() isn't that important for
preventing premature oom AFAICS, given it uses high wmark.

That said, I think the newly added check could be more logically placed
above the call to should_reclaim_retry() instead of right after the retry:
label, but it's not critical.

> zonelist change detection, so keeping the checks at
> https://elixir.bootlin.com/linux/v6.15-rc3/source/mm/page_alloc.c#L4652
> sounds like a good idea.
> 
>>
>> Therefore, I think retaining the original code logic is necessary to
>> ensure correctness under concurrency.
>>
>> >
>> >
>> >>>        /* Ensure kswapd doesn't accidentally go to sleep as long as we loop */
>> >>>        if (alloc_flags & ALLOC_KSWAPD)
>> >>>                wake_all_kswapds(order, gfp_mask, ac);
>> >>> --
>> >>> 2.20.1
>> >>>
>> >>>
>> Thanks
>>