Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup_flags for direct IO

Suren Baghdasaryan
2025-03-12T16:06:15Z

On Wed, Mar 12, 2025 at 8:52 AM Christoph Hellwig <hch@infradead.org> 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.

>