+ userfaultfd-shmem-allocate-anonymous-memory-for-map_private-shmem.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem
has been added to the -mm tree.  Its filename is
     userfaultfd-shmem-allocate-anonymous-memory-for-map_private-shmem.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/userfaultfd-shmem-allocate-anonymous-memory-for-map_private-shmem.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/userfaultfd-shmem-allocate-anonymous-memory-for-map_private-shmem.patch

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------------------------------------------------------
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem

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 anymore.

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 writable.

Link: http://lkml.kernel.org/r/20181126173452.26955-3-aarcange@redhat.com
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


--- a/mm/userfaultfd.c~userfaultfd-shmem-allocate-anonymous-memory-for-map_private-shmem
+++ a/mm/userfaultfd.c
@@ -380,7 +380,17 @@ static __always_inline ssize_t mfill_ato
 {
 	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 @@ retry:
 	 * 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) {
_

Patches currently in -mm which might be from aarcange@redhat.com are

userfaultfd-use-enoent-instead-of-efault-if-the-atomic-copy-user-fails.patch
userfaultfd-shmem-allocate-anonymous-memory-for-map_private-shmem.patch
userfaultfd-shmem-hugetlbfs-only-allow-to-register-vm_maywrite-vmas.patch
userfaultfd-shmem-add-i_size-checks.patch
userfaultfd-shmem-uffdio_copy-set-the-page-dirty-if-vm_write-is-not-set.patch