Re: [PATCH v2] mm: page_alloc: consume available CMA space first

[Andrew Morton]

On Wed, 26 Jul 2023 11:07:05 -0400 Johannes Weiner <hannes@xxxxxxxxxxx> wrote:

> On a memcache setup with heavy anon usage and no swap, we routinely
> see premature OOM kills with multiple gigabytes of free space left:
> 
>     Node 0 Normal free:4978632kB [...] free_cma:4893276kB
> 
> This free space turns out to be CMA. We set CMA regions aside for
> potential hugetlb users on all of our machines, figuring that even if
> there aren't any, the memory is available to userspace allocations.
> 
> When the OOMs trigger, it's from unmovable and reclaimable allocations
> that aren't allowed to dip into CMA. The non-CMA regions meanwhile are
> dominated by the anon pages.
> 
> Movable pages can be migrated out of CMA when necessary, but we don't
> have a mechanism to migrate them *into* CMA to make room for unmovable
> allocations. The only recourse we have for these pages is reclaim,
> which due to a lack of swap is unavailable in our case.
> 
> Because we have more options for CMA pages, change the policy to
> always fill up CMA first. This reduces the risk of premature OOMs.

This conflicts significantly (and more than textually) with "mm:
optimization on page allocation when CMA enabled", which has been
languishing in mm-unstable in an inadequately reviewed state since May
11.  Please suggest a way forward?