Re: [PATCH 14/31] fs/userfaultfd: retry if pte_offset_map() fails

[Peter Xu]

On Sun, May 21, 2023 at 10:06:32PM -0700, Hugh Dickins wrote:
> Instead of worrying whether the pmd is stable, userfaultfd_must_wait()
> call pte_offset_map() as before, but go back to try again if that fails.
> 
> Risk of endless loop?  It already broke out if pmd_none(), !pmd_present()
> or pmd_trans_huge(), and pte_offset_map() would have cleared pmd_bad():
> which leaves pmd_devmap().  Presumably pmd_devmap() is inappropriate in
> a vma subject to userfaultfd (it would have been mistreated before),
> but add a check just to avoid all possibility of endless loop there.

Agreed, afaiu that's for either PFNMAP or MIXEDMAP vmas only.  Maybe we can
use a WARN_ON_ONCE() for that to be clear, but no strong opinions.

> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Peter Xu <peterx@xxxxxxxxxx>

-- 
Peter Xu