linux-kernel - Re: [PATCH v1] mm/gup: fix handling of errors from arch_make_folio_accessible() in follow_page

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Message-ID: <20250910105715.7f4f8495@p-imbrenda>
Date: Wed, 10 Sep 2025 10:57:15 +0200
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton
 <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, John Hubbard
 <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Christian Borntraeger
 <borntraeger@...ibm.com>
Subject: Re: [PATCH v1] mm/gup: fix handling of errors from
 arch_make_folio_accessible() in follow_page_pte()

On Tue, 9 Sep 2025 18:15:17 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 09.09.25 18:07, Claudio Imbrenda wrote:
> > On Mon,  8 Sep 2025 11:45:17 +0200
> > David Hildenbrand <david@...hat.com> wrote:
> >   
> >> In case we call arch_make_folio_accessible() and it fails, we would
> >> incorrectly return a value that is "!= 0" to the caller, indicating that
> >> we pinned all requested pages and that the caller can keep going.
> >>
> >> follow_page_pte() is not supposed to return error values, but instead
> >> "0" on failure and "1" on success -- we'll clean that up separately.
> >>
> >> In case we return "!= 0", the caller will just keep going pinning
> >> more pages. If we happen to pin a page afterwards, we're in trouble,
> >> because we essentially skipped some pages in the requested range.
> >>
> >> Staring at the arch_make_folio_accessible() implementation on s390x, I
> >> assume it should actually never really fail unless something unexpected
> >> happens (BUG?). So let's not CC stable and just fix common code to do
> >> the right thing.
> >>
> >> Clean up the code a bit now that there is no reason to store the
> >> return value of arch_make_folio_accessible().
> >>
> >> Fixes: f28d43636d6f ("mm/gup/writeback: add callbacks for inaccessible pages")  
> > 
> > Ooops!
> > 
> > thanks for finding and fixing this  
> 
> Thanks! Is my assumption correct that this is not stable material?
> 

your assessment looks correct, an error return value can only be caused
by a bug