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Date:   Wed, 22 Nov 2023 23:25:28 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Rapoport <rppt@kernel.org>,
        Hugh Dickins <hughd@google.com>,
        David Hildenbrand <david@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Rik van Riel <riel@surriel.com>,
        James Houghton <jthoughton@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH RFC 05/12] mm/gup: Fix follow_devmap_p[mu]d() to return
 even if NULL
Message-ID: <ZV7+aP8x0aFE4D57@infradead.org>
References: <20231116012908.392077-1-peterx@redhat.com>
 <20231116012908.392077-6-peterx@redhat.com>
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On Wed, Nov 15, 2023 at 08:29:01PM -0500, Peter Xu wrote:
> This seems to be a bug not by any report but by code observations.
> 
> When GUP sees a devpmd or devpud, it should return whatever value returned
> from follow_devmap_p[mu]d().  If page==NULL returned, it means a fault is
> probably required.  Skipping return the NULL should allow the code to fall
> through, which can cause unexpected behavior.
> 
> It was there at least before the follow page rework (080dbb618b) in 2017,
> so 6 years.  Not yet digging for a Fixes, assuming it can hardly trigger
> even if the logical bug does exist.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

I'd still add a fixes tag and send if off to Linux for 6.7-rc instead
of letting it linger.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>