[PATCH v2 14/32] mm/userfaultfd: retry if pte_offset_map() fails

Hugh Dickins
2023-06-09T01:24:46Z

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.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
---
fs/userfaultfd.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f7a0817b1ec0..ca83423f8d54 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -349,12 +349,13 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
if (!pud_present(*pud))
goto out;
pmd = pmd_offset(pud, address);
+again:
_pmd = pmdp_get_lockless(pmd);
if (pmd_none(_pmd))
goto out;

ret = false;
- if (!pmd_present(_pmd))
+ if (!pmd_present(_pmd) || pmd_devmap(_pmd))
goto out;

if (pmd_trans_huge(_pmd)) {
@@ -363,11 +364,11 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
goto out;
}

- /*
- * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
- * and use the standard pte_offset_map() instead of parsing _pmd.
- */
pte = pte_offset_map(pmd, address);
+ if (!pte) {
+ ret = true;
+ goto again;
+ }
/*
* Lockless access: we're in a wait_event so it's ok if it
* changes under us. PTE markers should be handled the same as none
--
2.35.3