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

[Hugh Dickins <hughd@google.com>]

On Fri, 3 Jun 2022, 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>

Acked-by: Hugh Dickins <hughd@google.com>

Yes, that looks like a good improvement, and okay to call it a fix for
stable.  But note that your original choice of SGP_READ was not wrong:
there was no SGP_NOALLOC in v5.14, which arrived in v5.15 (and this
is not the first time that it's turned out to be useful since then).
Since v5.14-stable reached end-of-life some time ago, marking this
patch for stable should not result in any untoward build errors.

> ---
>  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) {
> -- 
> 2.36.1.255.ge46751e96f-goog