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From: Andrea Arcangeli <>
To: Andrew Morton <>
	Hugh Dickins <>,
	Mike Rapoport <>,
	Mike Kravetz <>,
	Jann Horn <>, Peter Xu <>,
	"Dr. David Alan Gilbert" <>
Subject: [PATCH 5/5] userfaultfd: shmem: UFFDIO_COPY: set the page dirty if VM_WRITE is not set
Date: Mon, 26 Nov 2018 12:34:52 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Set the page dirty if VM_WRITE is not set because in such case the pte
won't be marked dirty and the page would be reclaimed without
writepage (i.e. swapout in the shmem case).

This was found by source review. Most apps (certainly including QEMU)
only use UFFDIO_COPY on PROT_READ|PROT_WRITE mappings or the app can't
modify the memory in the first place. This is for correctness and it
could help the non cooperative use case to avoid unexpected data loss.

Reviewed-by: Hugh Dickins <>
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Reported-by: Hugh Dickins <>
Signed-off-by: Andrea Arcangeli <>
 mm/shmem.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index c3ece7a51949..82a381d463bc 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2272,6 +2272,16 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
 	if (dst_vma->vm_flags & VM_WRITE)
 		_dst_pte = pte_mkwrite(pte_mkdirty(_dst_pte));
+	else {
+		/*
+		 * We don't set the pte dirty if the vma has no
+		 * VM_WRITE permission, so mark the page dirty or it
+		 * could be freed from under us. We could do it
+		 * unconditionally before unlock_page(), but doing it
+		 * only if VM_WRITE is not set is faster.
+		 */
+		set_page_dirty(page);
+	}
 	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
@@ -2305,6 +2315,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	return ret;
 	pte_unmap_unlock(dst_pte, ptl);
+	ClearPageDirty(page);
 	mem_cgroup_cancel_charge(page, memcg, false);

      parent reply	other threads:[~2018-11-26 17:35 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-26 17:34 [PATCH 0/5] userfaultfd shmem updates Andrea Arcangeli
2018-11-26 17:34 ` [PATCH 1/5] userfaultfd: use ENOENT instead of EFAULT if the atomic copy user fails Andrea Arcangeli
2018-11-26 17:34 ` [PATCH 2/5] userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem Andrea Arcangeli
2018-11-26 17:34 ` [PATCH 3/5] userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas Andrea Arcangeli
2018-11-26 17:34 ` [PATCH 4/5] userfaultfd: shmem: add i_size checks Andrea Arcangeli
     [not found]   ` <>
2018-11-27 10:00     ` Mike Rapoport
2018-11-26 17:34 ` Andrea Arcangeli [this message]

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