linux-kernel - Re: [PATCH RFC 05/12] mm/gup: Fix follow_devmap

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Date:   Thu, 23 Nov 2023 12:59:27 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Rapoport <rppt@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        James Houghton <jthoughton@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RFC 05/12] mm/gup: Fix follow_devmap_p[mu]d() to return
 even if NULL

On Wed, Nov 22, 2023 at 11:25:28PM -0800, Christoph Hellwig wrote:
> 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@...hat.com>
> 
> I'd still add a fixes tag and send if off to Linux for 6.7-rc instead
> of letting it linger.

Will do.

> 
> Otherwise looks good:
> 
> Reviewed-by: Christoph Hellwig <hch@....de>

When rethinking, "return page" is correct for devmap, but it should be
better to always use no_page_table() instead for page==NULL when trying to
return NULL for follow_page(), even if no_page_table() should constantly
return NULL for devmap, afaict, so it should be the same.

So I'll make it slightly different when reposting separately, please feel
free to have another look.

Thanks,

-- 
Peter Xu