linux-kernel - Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup

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Message-ID: <Z9Gty3Ax-2RslqDX@infradead.org>
Date: Wed, 12 Mar 2025 08:52:43 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Christoph Hellwig <hch@...radead.org>, Sooyong Suk <s.suk@...sung.com>,
	Jaewon Kim <jaewon31.kim@...il.com>, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, spssyr@...il.com, axboe@...nel.dk,
	linux-block@...r.kernel.org, dhavale@...gle.com
Subject: Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup_flags for direct
 IO

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.

> 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?