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Date:   Fri, 28 Apr 2023 18:42:41 +0100
From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     David Hildenbrand <david@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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 <ZEv2196tk5yWvgW5@x1n>
 <173337c0-14f4-3246-15ff-7fbf03861c94@redhat.com>
 <20230428165623.pqchgi5gtfhxd5b5@box.shutemov.name>
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On Fri, Apr 28, 2023 at 02:31:38PM -0300, Jason Gunthorpe wrote:
> On Fri, Apr 28, 2023 at 07:02:22PM +0200, David Hildenbrand wrote:
>
> > > No. VMA cannot get away before PTEs are unmapped and TLB is flushed. And
> > > TLB flushing is serialized against GUP_fast().
> >
> > The whole CONFIG_MMU_GATHER_RCU_TABLE_FREE handling makes the situation more
> > complicated.
>
> Yeah, you have to think of gup_fast as RCU with a hacky pre-RCU implementation
> on most architectures.
>
> We could make page->mapping safe under RCU, for instance.
>
> Jason

Does it really require a change though? I might be missing some details,
but afaict with interrupts disabled we should be ok to deref page->mapping
to check PageAnon and a_ops before handing back a page right?