Re: [PATCH v1] mm/gup: fix handling of errors from arch_make_folio_accessible() in follow_page_pte()

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

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

> On 09.09.25 18:07, Claudio Imbrenda wrote:
> > On Mon,  8 Sep 2025 11:45:17 +0200
> > David Hildenbrand <david@redhat.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