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    SubjectRe: mm: fault in __do_fault
    Hello Dmitry,

    On Tue, Feb 28, 2017 at 03:04:53PM +0100, Dmitry Vyukov wrote:
    > Hello,
    >
    > The following program triggers GPF in __do_fault:
    > https://gist.githubusercontent.com/dvyukov/27345737fca18d92ef761e7fa08aec9b/raw/d99d02511d0bf9a8d6f6bd9c79d373a26924e974/gistfile1.txt

    Can you verify this fix:

    From a65381bc86d2963713b6a9c4a73cded7dd184282 Mon Sep 17 00:00:00 2001
    From: Andrea Arcangeli <aarcange@redhat.com>
    Date: Tue, 28 Feb 2017 16:36:59 +0100
    Subject: [PATCH 1/1] userfaultfd: shmem: __do_fault requires VM_FAULT_NOPAGE

    __do_fault assumes vmf->page has been initialized and is valid if
    VM_FAULT_NOPAGE is not returned by vma->vm_ops->fault(vma, vmf).

    handle_userfault() in turn should return VM_FAULT_NOPAGE if it doesn't
    return VM_FAULT_SIGBUS or VM_FAULT_RETRY (the other two
    possibilities).

    This VM_FAULT_NOPAGE case is only invoked when signal are pending and
    it didn't matter for anonymous memory before. It only started to
    matter since shmem was introduced. hugetlbfs also takes a different
    path and doesn't exercise __do_fault.

    Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
    ---
    fs/userfaultfd.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
    index fb6d02b..de28f43 100644
    --- a/fs/userfaultfd.c
    +++ b/fs/userfaultfd.c
    @@ -500,7 +500,7 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
    * in such case.
    */
    down_read(&mm->mmap_sem);
    - ret = 0;
    + ret = VM_FAULT_NOPAGE;
    }
    }

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