On Wed, Jan 29, 2025 at 8:10 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > Hi, > > ___GFP_MOVABLE allocations are supposed to be movable -> migratable: the > page allocator can place them on > MIGRATE_CMA/ZONE_MOVABLE/MIGRATE_MOVABLE areas: areas where the > expectation is that allocations can be migrated (somewhat reliably) to > different memory areas on demand. > > Mechanisms that turn such allocations unmigratable, such as long-term > page pinning (FOLL_LONGTERM), migrate these allocations at least out of > MIGRATE_CMA/ZONE_MOVABLE areas first. > > Ideally, we'd only perform this migration if really required (e.g., > long-term pinning), and rather "fix" other cases to not turn allocations > unmigratable. > > However, we have some rather obscure cases that can turn migratable > allocations effectively unmigratable for a long/indeterminate time, > possibly controlled by unprivileged user space. > > Possible effects include: > * CMA allocations failing > * Memory hotunplug not making progress > * Memory compaction not working as expected > > Some cases I can fix myself [1], others are harder to tackle. > > As one example, in context of FUSE we recently discovered that folios > that are under writeback cannot be migrated, and user space in control > of when writeback will end. Something similar can happen ->readahead() > where user space is in charge of supplying page content. Networking > filesystems in general seem to be prone to this as well. > > As another example, failing to split large folios can prevent migration > if memory is fragmented. XFS (IOMAP in general) refuses to split folios > that are dirty [3]. Splitting of folios and page migration have a lot in > common. > > This session is to collect cases that are known to be problematic, and > to start discussing possible approaches to make some of these > un-migratable allocations migratable, or alternative strategies to deal > with this. > > > [1] https://lkml.kernel.org/r/20250129115411.2077152-1-david@xxxxxxxxxx > [2] > https://lkml.kernel.org/r/CAJnrk1ZCgff6ZWmqKzBXFq5uAEbms46OexA1axWS5v-PCZFqJg@xxxxxxxxxxxxxx > [3] > https://lkml.kernel.org/r/4febc035-a4ff-4afe-a9a0-d127826852a9@xxxxxxxxxx > > -- > Cheers, > > David / dhildenb > > We have run in to the same issues (especially the writeback one), so a definite +1 on this topic from me. - Frank
