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Date:   Tue, 25 Apr 2023 17:45:36 +0100
From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     "Kirill A . Shutemov" <kirill@shutemov.name>
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Subject: Re: [PATCH v3] mm/gup: disallow GUP writing to file-backed mappings
 by default
Message-ID: <ZEgDsOlIW1xhuQXv@murray>
References: <23c19e27ef0745f6d3125976e047ee0da62569d4.1682406295.git.lstoakes@gmail.com>
 <20230425101153.xxi4arpwkz7ijnvm@box.shutemov.name>
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On Tue, Apr 25, 2023 at 01:11:53PM +0300, Kirill A . Shutemov wrote:
> On Tue, Apr 25, 2023 at 08:14:14AM +0100, Lorenzo Stoakes wrote:
> > GUP does not correctly implement write-notify semantics, nor does it
> > guarantee that the underlying pages are correctly dirtied, which could lead
> > to a kernel oops or data corruption when writing to file-backed mappings.
> >
> > This is only relevant when the mappings are file-backed and the underlying
> > file system requires folio dirty tracking. File systems which do not, such
> > as shmem or hugetlb, are not at risk and therefore can be written to
> > without issue.
> >
> > Unfortunately this limitation of GUP has been present for some time and
> > requires future rework of the GUP API in order to provide correct write
> > access to such mappings.
> >
> > In the meantime, we add a check for the most broken GUP case -
> > FOLL_LONGTERM - which really under no circumstances can safely access
> > dirty-tracked file mappings.
> >
> > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > ---
> > v3:
> > - Rebased on latest mm-unstable as of 24th April 2023.
> > - Explicitly check whether file system requires folio dirtying. Note that
> >   vma_wants_writenotify() could not be used directly as it is very much focused
> >   on determining if the PTE r/w should be set (e.g. assuming private mapping
> >   does not require it as already set, soft dirty considerations).
>
> Hm. Okay. Have you considered having a common base for your case and
> vma_wants_writenotify()? Code duplication doesn't look good.
>

I did and I actually started implementing something for the same reason,
however I wondered whether it was worth it for essentially 3 clauses that
are shared between the two.

On second thoughts, it is painful to have this duplicated, so let me take
another look.

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov