[PATCH v4 2/6] mm: gup: clarify a reason to check pte/pmd in gup_fast_pte_range()

[Yeoreum Yun]

Clarify a reason to check pte/pmd in gup_fast_pte_range().

Reviewed-by: Rik van Riel <riel@xxxxxxxxxxx>
Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
---
 mm/gup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 2b405cce2ca5..00dac03093a4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2885,6 +2885,14 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp,
 		if (!folio)
 			goto pte_unmap;
 
+		/*
+		 * Recheck the PTE after grabbing the folio to detect concurrent
+		 * unmapping or protection changes such as by KSM.
+		 *
+		 * Also, recheck the PMD: THP collapse can detach the PTE table
+		 * while its entries remain unchanged. Checking the PTE alone
+		 * would not detect that we are walking a detached page table.
+		 */
 		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);

-- 
2.43.0