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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 2/5] userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem
Date: Mon, 26 Nov 2018 12:34:49 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Userfaultfd did not create private memory when UFFDIO_COPY was invoked
on a MAP_PRIVATE shmem mapping. Instead it wrote to the shmem file,
even when that had not been opened for writing. Though, fortunately,
that could only happen where there was a hole in the file.

Fix the shmem-backed implementation of UFFDIO_COPY to create private
memory for MAP_PRIVATE mappings. The hugetlbfs-backed implementation
was already correct.

This change is visible to userland, if userfaultfd has been used in
unintended ways: so it introduces a small risk of incompatibility, but
is necessary in order to respect file permissions.

An app that uses UFFDIO_COPY for anything like postcopy live migration
won't notice the difference, and in fact it'll run faster because
there will be no copy-on-write and memory waste in the tmpfs pagecache

Userfaults on MAP_PRIVATE shmem keep triggering only on file holes
like before.

The real zeropage can also be built on a MAP_PRIVATE shmem mapping
through UFFDIO_ZEROPAGE and that's safe because the zeropage pte is
never dirty, in turn even an mprotect upgrading the vma permission
from PROT_READ to PROT_READ|PROT_WRITE won't make the zeropage pte

Reported-by: Mike Rapoport <>
Reviewed-by: Hugh Dickins <>
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Andrea Arcangeli <>
 mm/userfaultfd.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 46c8949e5f8f..471b6457f95f 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -380,7 +380,17 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 	ssize_t err;
-	if (vma_is_anonymous(dst_vma)) {
+	/*
+	 * The normal page fault path for a shmem will invoke the
+	 * fault, fill the hole in the file and COW it right away. The
+	 * result generates plain anonymous memory. So when we are
+	 * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll
+	 * generate anonymous memory directly without actually filling
+	 * the hole. For the MAP_PRIVATE case the robustness check
+	 * only happens in the pagetable (to verify it's still none)
+	 * and not in the radix tree.
+	 */
+	if (!(dst_vma->vm_flags & VM_SHARED)) {
 		if (!zeropage)
 			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
 					       dst_addr, src_addr, page);
@@ -489,7 +499,8 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 	 * dst_vma.
 	err = -ENOMEM;
-	if (vma_is_anonymous(dst_vma) && unlikely(anon_vma_prepare(dst_vma)))
+	if (!(dst_vma->vm_flags & VM_SHARED) &&
+	    unlikely(anon_vma_prepare(dst_vma)))
 		goto out_unlock;
 	while (src_addr < src_start + len) {

  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 ` Andrea Arcangeli [this message]
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 ` [PATCH 5/5] userfaultfd: shmem: UFFDIO_COPY: set the page dirty if VM_WRITE is not set Andrea Arcangeli

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