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

[Peter Xu]

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@xxxxxxxxxxxxxxx
> Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> ---
>  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