[merged mm-hotfixes-stable] mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch removed from -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The quilt patch titled
     Subject: mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages
has been removed from the -mm tree.  Its filename was
     mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Axel Rasmussen <axelrasmussen@google.com>
Subject: mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages
Date: Fri, 10 Jun 2022 10:38:12 -0700

When fallocate() is used on a shmem file, the pages we allocate can end up
with !PageUptodate.

Since UFFDIO_CONTINUE tries to find the existing page the user wants to
map with SGP_READ, we would fail to find such a page, since
shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it discovers
!PageUptodate.  As a result, UFFDIO_CONTINUE returns -EFAULT, as it would
do if the page wasn't found in the page cache at all.

This isn't the intended behavior.  UFFDIO_CONTINUE is just trying to find
if a page exists, and doesn't care whether it still needs to be cleared or
not.  So, instead of SGP_READ, pass in SGP_NOALLOC.  This is the same,
except for one critical difference: in the !PageUptodate case, SGP_NOALLOC
will clear the page and then return it.  With this change, UFFDIO_CONTINUE
works properly (succeeds) on a shmem file which has been fallocated, but
otherwise not modified.

Link: https://lkml.kernel.org/r/20220610173812.1768919-1-axelrasmussen@google.com
Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem")
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/userfaultfd.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/userfaultfd.c~mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages
+++ a/mm/userfaultfd.c
@@ -246,7 +246,10 @@ static int mcontinue_atomic_pte(struct m
 	struct page *page;
 	int ret;
 
-	ret = shmem_getpage(inode, pgoff, &page, SGP_READ);
+	ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC);
+	/* Our caller expects us to return -EFAULT if we failed to find page. */
+	if (ret == -ENOENT)
+		ret = -EFAULT;
 	if (ret)
 		goto out;
 	if (!page) {
_

Patches currently in -mm which might be from axelrasmussen@google.com are

selftests-vm-add-hugetlb_shared-userfaultfd-test-to-run_vmtestssh.patch
userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control.patch
userfaultfd-selftests-modify-selftest-to-use-dev-userfaultfd.patch
userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch
userfaultfd-selftests-make-dev-userfaultfd-testing-configurable.patch
selftests-vm-add-dev-userfaultfd-test-cases-to-run_vmtestssh.patch