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Date:   Fri, 28 Apr 2023 14:49:22 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Lorenzo Stoakes <lstoakes@gmail.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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On Fri, Apr 28, 2023 at 06:42:41PM +0100, Lorenzo Stoakes wrote:
> 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?
 
AFAIK not on all architectures

Jason