[PATCH v9 24/32] mm/madvise: keep PMD swap entries whole for MADV_GUARD_INSTALL/REMOVE

[Usama Arif <usama.arif@linux.dev>]

Both callbacks test bare pmd_trans_huge(), which is false for a PMD swap
entry, so they return without consuming the PMD and walk_pmd_range() splits
it - only for the PTE callbacks to find the range populated and abort, or
to find no guard markers and do nothing.

Both already short-circuit at PMD level for a present THP. Do the same for
a swapped-out one: it is just as populated, and just as free of guard
markers.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/madvise.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 576707e303cf2..f7f17efdd926d 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1109,8 +1109,12 @@ static int guard_install_pmd_entry(pmd_t *pmd, unsigned long addr,
 {
 	pmd_t pmdval = pmdp_get(pmd);
 
-	/* If huge return >0 so we abort the operation + zap. */
-	return pmd_trans_huge(pmdval);
+	/*
+	 * If huge return >0 so we abort the operation + zap. A PMD swap entry
+	 * is a swapped-out THP: also populated, and splitting it here would
+	 * only demote it before the zap.
+	 */
+	return pmd_trans_huge(pmdval) || pmd_is_swap_entry(pmdval);
 }
 
 static int guard_install_pte_entry(pte_t *pte, unsigned long addr,
@@ -1249,8 +1253,12 @@ static int guard_remove_pmd_entry(pmd_t *pmd, unsigned long addr,
 {
 	pmd_t pmdval = pmdp_get(pmd);
 
-	/* If huge, cannot have guard pages present, so no-op - skip. */
-	if (pmd_trans_huge(pmdval))
+	/*
+	 * If huge, cannot have guard pages present, so no-op - skip. The same
+	 * is true of a PMD swap entry, which must not be split just to discover
+	 * there is nothing to remove.
+	 */
+	if (pmd_trans_huge(pmdval) || pmd_is_swap_entry(pmdval))
 		walk->action = ACTION_CONTINUE;
 
 	return 0;
-- 
2.53.0-Meta