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    SubjectRe: [PATCH v2 13/15] mm: support GUP-triggered unsharing of anonymous pages
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    On 19.03.22 00:30, Jason Gunthorpe wrote:
    > On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote:
    >> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
    >> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
    >> on the page table entry will end up replacing the mapped anonymous page
    >> due to COW, resulting in the GUP pin no longer being consistent with the
    >> page actually mapped into the page table.
    >>
    >> The possible ways to deal with this situation are:
    >> (1) Ignore and pin -- what we do right now.
    >> (2) Fail to pin -- which would be rather surprising to callers and
    >> could break user space.
    >> (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
    >> pins.
    >>
    >> We want to implement 3) because it provides the clearest semantics and
    >> allows for checking in unpin_user_pages() and friends for possible BUGs:
    >> when trying to unpin a page that's no longer exclusive, clearly
    >> something went very wrong and might result in memory corruptions that
    >> might be hard to debug. So we better have a nice way to spot such
    >> issues.
    >>
    >> To implement 3), we need a way for GUP to trigger unsharing:
    >> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped
    >> anonymous pages and resembles COW logic during a write fault. However, in
    >> contrast to a write fault, GUP-triggered unsharing will, for example, still
    >> maintain the write protection.
    >
    > Given the way this series has developed you might want to call this
    > FAULT_FLAG_MAKE_ANON_EXCLUSIVE
    >
    > Which strikes me as more directly connected to what it is trying to
    > do.

    I thought about something similar along those lines, and I think it
    would apply even when extending that mechanism to anything !anon inside
    a MAP_PRIVATE mapping.

    The whole

    const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;

    would get a little more verbose, though

    const bool make_anon_exclusive = vmf->flags &
    FAULT_FLAG_MAKE_ANON_EXCLUSIVE;


    No strong opinion, any other opinions or alternatives?

    --
    Thanks,

    David / dhildenb

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