[RFC PATCH 2/2] mm/gup: add lockless access semantics on entries validation

[Alexander Gordeev]

The PTE validation in gup_fast_pte_range() is inconsistent with the
prior value acquisition in the sense that it drops the lockless
access semantics.

Although this is highly unlikely, prevent a future scenario in which
a semantically mismatching ptep_get() incorrectly yields the same
result as the preceding ptep_get_lockless(), while ptep_get_lockless()
would otherwise return a different value.

Likewise the PMD validation is inconsistent with the prior value
acquisition in gup_fast_pmd_range().

Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
---
 mm/gup.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index d149a4b0df71..236450feea9a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2865,8 +2865,8 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
 		if (!folio)
 			goto pte_unmap;
 
-		if (unlikely(pmd_val(pmd) != pmd_val(pmdp_get(pmdp))) ||
-		    unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
+		if (unlikely(pmd_val(pmd) != pmd_val(pmdp_get_lockless(pmdp))) ||
+		    unlikely(pte_val(pte) != pte_val(ptep_get_lockless(ptep)))) {
 			gup_put_folio(folio, 1, flags);
 			goto pte_unmap;
 		}
@@ -2942,7 +2942,7 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	if (!folio)
 		return 0;
 
-	if (unlikely(pmd_val(orig) != pmd_val(pmdp_get(pmdp)))) {
+	if (unlikely(pmd_val(orig) != pmd_val(pmdp_get_lockless(pmdp)))) {
 		gup_put_folio(folio, refs, flags);
 		return 0;
 	}
-- 
2.51.0