Re: [PATCH v2] mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages

[Peter Xu <peterx@redhat.com>]

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,

-- 
Peter Xu