Re: [PATCH] mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages

[Axel Rasmussen <axelrasmussen@google.com>]

Ah, thanks for pointing this out Peter, it is definitely something I missed.

You're right that elsewhere in mm/userfaultfd.c we use -ENOENT for
some special case. I think for mcontinue_atomic_pte, we don't want to
change the status quo - if we fail to lookup an existing page, we
should just return -EFAULT just like we were doing before. We
certainly shouldn't return -ENOENT, as that causes us to take a wrong,
unrelated code path a couple of callers up, as you mentioned.

I'll send a v2 with this small modification.

On Thu, Jun 9, 2022 at 2:44 PM Peter Xu <peterx@redhat.com> wrote:
>
> 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,
>
> --
> Peter Xu
>