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...

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@google.com Subject: mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages Date: Fri, 3 Jun 2022 13:57:41 -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/20220603205741.12888-1-axelrasmussen@google.com Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") Signed-off-by: Axel Rasmussen axelrasmussen@google.com Cc: 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 | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/userfaultfd.c~mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages +++ a/mm/userfaultfd.c @@ -246,7 +246,7 @@ 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); if (ret) goto out; if (!page) { _

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

mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch