linux-kernel - Re: [PATCH] mm: gup: fail migration when no migratable page to prevent CMA pinning

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Message-ID: <20250604131158.GA17991@ziepe.ca>
Date: Wed, 4 Jun 2025 10:11:58 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: David Hildenbrand <david@...hat.com>
Cc: Hyesoo Yu <hyesoo.yu@...sung.com>, janghyuck.kim@...sung.com,
	zhaoyang.huang@...soc.com, jaewon31.kim@...il.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Hubbard <jhubbard@...dia.com>, Peter Xu <peterx@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: gup: fail migration when no migratable page to
 prevent CMA pinning

On Wed, Jun 04, 2025 at 12:07:21PM +0200, David Hildenbrand wrote:

> > Instead of retrying, this patch explicitly fails the migration attempt
> > (-EBUSY) if no movable pages are found and unpinnable pages remain.
> > This avoids infinite loops and gives user a clear signal to retry,
> > rather then spinning inside kernel.
> 
> Hmmm, that means we will return EBUSY to the caller. Are all users actually
> prepared to deal with that?

I don't think anyone is really prepared to deal with GUP temporarily
failing..

Kernel is expected to sort it out. We tolerated the existing temporary
failure due to its rarity and lack of a solution only.

Either it can be gup'd or not. There should be no retry.

Jason