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

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Message-ID: <CAJuCfpGa43OQHG9BmnvxROX1AneCvkuLxFwM+TdxAdR1v9kWSg@mail.gmail.com>
Date: Wed, 12 Mar 2025 08:20:36 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: 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 8:17 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> 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.

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.

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