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.
Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") Cc: stable@vger.kernel.org Signed-off-by: Axel Rasmussen axelrasmussen@google.com --- mm/userfaultfd.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 4f4892a5f767..07d3befc80e4 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -246,7 +246,10 @@ static int mcontinue_atomic_pte(struct mm_struct *dst_mm, 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) {