[PATCH v8 24/30] mm/khugepaged: treat PMD swap entries as mapped THPs

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

check_pmd_state() would report a PMD swap entry as SCAN_NO_PTE_TABLE - it
is not present and, on x86, is also pmd_bad(). The range is already a THP;
it just happens to be on disk.

Return SCAN_PMD_MAPPED next to the existing migration-entry case and before
the pmd_present() and pmd_bad() tests.

This does change MADV_COLLAPSE. Today a swapped-out THP is HPAGE_PMD_NR PTE
swap entries and MADV_COLLAPSE, which ignores khugepaged_max_ptes_swap,
swaps them all in. With a PMD swap entry it reports SCAN_PMD_MAPPED, which
madvise_collapse() counts as success, so the call returns 0 without making
the range resident. It still faults back in as a whole THP on first touch;
only the point at which the I/O happens moves.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/khugepaged.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0d8cd44b14c57..bbbf96c616295 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1052,6 +1052,12 @@ static inline enum scan_result check_pmd_state(pmd_t *pmd)
 	 */
 	if (pmd_is_migration_entry(pmde))
 		return SCAN_PMD_MAPPED;
+	/*
+	 * A PMD-mapped THP that has been swapped out is still a THP from
+	 * khugepaged's perspective; treat it like a present huge PMD.
+	 */
+	if (pmd_is_swap_entry(pmde))
+		return SCAN_PMD_MAPPED;
 	if (!pmd_present(pmde))
 		return SCAN_NO_PTE_TABLE;
 	if (pmd_trans_huge(pmde))
-- 
2.53.0-Meta