Hi, Axel,
Sorry to read this late.
On Fri, Jun 03, 2022 at 01:57:41PM -0700, Axel Rasmussen wrote:
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 | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 4f4892a5f767..c156f7f5b854 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -246,7 +246,7 @@ 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); if (ret) goto out; if (!page) {
It all looks sane if the page is !uptodate as you described. Though I've a question on what'll happen if the page is actually missing rather than just !PageUptodate().
My reading is previously it'll keep returning 0 on shmem_getpage_gfp() for both cases, but now for the missing page shmem_getpage_gfp() will return -ENOENT instead.
This reminded me on whether this will errornously let __mcopy_atomic() go into the special path to copy the page without mmap lock, please see this commit:
b6ebaedb4cb1 ("userfaultfd: avoid mmap_sem read recursion in mcopy_atomic", 2015-09-04)
Would that be a problem? Or could I read it wrong?
This also reminded me that whether we'd better need some protection in the -ENOENT handling in __mcopy_atomic() to be always safe.
Thanks,