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?
