The patch titled
Subject: mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
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@xxxxxxxxxx
Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem")
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxx are
mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch
