On Wed, Jul 26, 2023 at 10:53:04AM -0400, Johannes Weiner 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. > > > Because we have more options for CMA pages, change the policy to > always fill up CMA first. This reduces the risk of premature OOMs. I suspect it might cause regressions on small(er) devices where a relatively small cma area (Mb's) is often reserved for a use by various device drivers, which can't handle allocation failures well (even interim allocation failures). A startup time can regress too: migrating pages out of cma will take time. And given the velocity of kernel upgrades on such devices, we won't learn about it for next couple of years. > 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. Idk, should we introduce such a mechanism? Or use some alternative heuristics, which will be a better compromise between those who need cma allocations always pass and those who use large cma areas for opportunistic huge page allocations. Of course, we can add a boot flag/sysctl/per-cma-area flag, but I doubt we want really this. Thanks!
