6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Gregory Price [ Upstream commit 5a6c9cf117fca3c38a98fb2a2d5549795c8b0b73 ] MADV_COLD or MADV_PAGEOUT over part of a PMD splits the THP in madvise_cold_or_pageout_pte_range(). Two threads doing that to the same THP create spurious failures. CPU0 CPU1 ---- ---- folio_get() spin_unlock(ptl) folio_lock() folio_get() spin_unlock(ptl) folio_lock() <- blocks, keeps its ref split_folio() folio_expected_ref_count(folio) != folio_ref_count(folio) - 1 -EAGAIN CPU1 cannot drop its reference until it gets the lock CPU0 holds, so CPU0's split always fails. folio_trylock() makes CPU1 leave without ever taking a reference. The PTE branch of this same function already does this, as do madvise_free_pte_range() and madvise_free_huge_pmd(). Reproducer: 400 rounds of eight threads calling MADV_COLD on half of each of eight THPs, re-formed with MADV_COLLAPSE between rounds. From /proc/vmstat: thp_split_page thp_split_page_failed before 3186 860 after 3200 0 The short before count is rounds where every thread failed and the advice was dropped for that THP entirely. On failure the walker returns 0 and nothing retries. The PMD path becomes best effort when the folio lock is held elsewhere - same as the PTE path. Link: https://lore.kernel.org/20260912110540.3203010-1-gourry@gourry.net Signed-off-by: Gregory Price (Meta) Signed-off-by: Andrew Morton Reported-by: sashiko-bot Closes: https://sashiko.dev/#/patchset/20260817220810.1175596-1-gourry%40gourry.net Acked-by: Lorenzo Stoakes (ARM) Acked-by: David Hildenbrand (Arm) Assisted-by: LLM Cc: Jann Horn Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Signed-off-by: Sasha Levin --- mm/madvise.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/madvise.c b/mm/madvise.c index 6b62e01f31911..505a580cd9c4c 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -415,9 +415,10 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, if (next - addr != HPAGE_PMD_SIZE) { int err; + if (!folio_trylock(folio)) + goto huge_unlock; folio_get(folio); spin_unlock(ptl); - folio_lock(folio); err = split_folio(folio); folio_unlock(folio); folio_put(folio); -- 2.53.0