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Howlett" , Vlastimil Babka , Sasha Levin Subject: [PATCH 5.15.y] mm/madvise: use folio_trylock() in the cold/pageout PMD split Date: Tue, 10 Nov 2026 09:42:26 -0500 Message-ID: <20261110144208.1960054-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261107060446-failed-5.15-5a6c9cf117fc@kernel.org> References: <20261107060446-failed-5.15-5a6c9cf117fc@kernel.org> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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: [ sashal: replaced folio_trylock(folio) with trylock_page(page) to match the existing page-based API ] 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 367220e1fe52a..b19ed034e84f4 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -351,9 +351,10 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, if (next - addr != HPAGE_PMD_SIZE) { int err; + if (!trylock_page(page)) + goto huge_unlock; get_page(page); spin_unlock(ptl); - lock_page(page); err = split_huge_page(page); unlock_page(page); put_page(page); -- 2.53.0