| Message ID | 20220610173812.1768919-1-axelrasmussen@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages | expand |
On Fri, Jun 10, 2022 at 10:38:12AM -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 | 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; I checked shmem_getpage_gfp() and it only returns -ENOENT in one place where is the missing case for SGP_NOALLOC, then this looks correct to me. Acked-by: Peter Xu <peterx@redhat.com> Thanks,
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) {
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(-)