On Wed, Mar 12, 2025 at 8:52 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > On Wed, Mar 12, 2025 at 08:38:07AM -0700, Suren Baghdasaryan wrote: > > I might be wrong but my understanding is that we should try to > > allocate from CMA when the allocation is movable (not pinned), so that > > CMA can move those pages if necessary. I understand that in some cases > > a movable allocation can be pinned and we don't know beforehand > > whether it will be pinned or not. But in this case we know it will > > happen and could avoid this situation. > > Any file or anonymous folio can be temporarily pinned for I/O and only > moved once that completes. Direct I/O is one use case for that but there > are plenty others. I'm not sure how you define "beforehand", but the > pinning is visible in the _pincount field. Well, by "beforehand" I mean that when allocating for Direct I/O operation we know this memory will be pinned, so we could tell the allocator to avoid CMA. However I agree that FOLL_LONGTERM is a wrong way to accomplish that. > > > Yeah, low latency usecases for CMA are problematic and I think the > > only current alternative (apart from solutions involving HW change) is > > to use a memory carveouts. Device vendors hate that since carved-out > > memory ends up poorly utilized. I'm working on a GCMA proposal which > > hopefully can address that. > > I'd still like to understand what the use case is. Who does CMA > allocation at a time where heavy direct I/O is in progress? I'll let Samsung folks clarify their usecase. >
