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    Subject[RFC PATCH 02/14] userfaultfd: try to map write-unprotected pages
    Date
    From: Nadav Amit <namit@vmware.com>

    When using userfaultfd write-(un)protect ioctl, try to change the PTE to
    be writable. This would save a page-fault afterwards.

    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Andrew Cooper <andrew.cooper3@citrix.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: David Hildenbrand <david@redhat.com>
    Cc: Peter Xu <peterx@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Will Deacon <will@kernel.org>
    Cc: Yu Zhao <yuzhao@google.com>
    Cc: Nick Piggin <npiggin@gmail.com>
    Signed-off-by: Nadav Amit <namit@vmware.com>
    ---
    mm/userfaultfd.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
    index e0492f5f06a0..6013b217e9f3 100644
    --- a/mm/userfaultfd.c
    +++ b/mm/userfaultfd.c
    @@ -799,6 +799,8 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
    cp_flags = enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE;
    if (uffd_flags & (UFFD_FLAGS_ACCESS_LIKELY|UFFD_FLAGS_WRITE_LIKELY))
    cp_flags |= MM_CP_WILL_NEED;
    + if (!enable_wp && (uffd_flags & UFFD_FLAGS_WRITE_LIKELY))
    + cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;

    tlb_gather_mmu(&tlb, dst_mm);
    change_protection(&tlb, dst_vma, start, start + len, newprot, cp_flags);
    --
    2.25.1
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