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

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Message-ID: <Z9GnUaUD-iaFre_i@infradead.org>
Date: Wed, 12 Mar 2025 08:25:05 -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 08:20:36AM -0700, Suren Baghdasaryan wrote:
> > Direct I/O pages are not unmovable.  They are temporarily pinned for
> > the duration of the direct I/O.
> 
> Yes but even temporarily pinned pages can cause CMA allocation
> failure. My point is that if we know beforehand that the pages will be
> pinned we could avoid using CMA and these failures would go away.

Direct I/O (and other users of pin_user_pages) are designed to work
on all anonymous and file backed pages, which is kinda the point.
If you CMA user can't wait for the time of an I/O something is wrong
with that caller and it really should not use CMA.