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

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Message-ID: <Z9Gld_s3XYic8-dG@infradead.org>
Date: Wed, 12 Mar 2025 08:17:11 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Sooyong Suk <s.suk@...sung.com>, Jaewon Kim <jaewon31.kim@...il.com>,
	Christoph Hellwig <hch@...radead.org>, 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 Thu, Mar 06, 2025 at 06:28:40PM -0800, Suren Baghdasaryan wrote:
> I think this will help you only when the pages are faulted in but if
> __get_user_pages() finds an already mapped page which happens to be
> allocated from CMA, it will not migrate it. So, you might still end up
> with unmovable pages inside CMA.

Direct I/O pages are not unmovable.  They are temporarily pinned for
the duration of the direct I/O.

I really don't understand what problem you're trying to fix here.