From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 909332F0C52; Mon, 22 Sep 2025 19:43:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758570232; cv=none; b=CDcr1psYA6z+cpiChfHrPBGX0y3rKA2dF3DTyUi8hAiXZ51mZEJfw0ZQTuAM86HCrwNbs2qTLBxfPJYTxYiIabqcB9hVeNCoYAXSS9ll4tHNZULlGd3blhPueAvFqoVRyQqlS1RboNPl4MqTk7sJ2JVSDTQXeHlw1hsfk02BMKE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758570232; c=relaxed/simple; bh=TJpR6oruumCYfPwA3xSw7LppxUymQtsaKp7hkKBlGFc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=c/RUbYlH9MSFLbTG0Bu3ghJBN9FkxTbigEhqvFOyBmWDSk+pLm88S5DMaZP0IMHh2n7U7uisNqLABGAvkd/U0j8YZymQtYEq1QDCXA2CTF5xLlgvpf20xtu4rAV7qf6ViSSKYntkE6FWVptjLiE0xr+rFTLTO48z5gX329IkCZU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=nBrBMnki; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="nBrBMnki" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0272FC4CEF0; Mon, 22 Sep 2025 19:43:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1758570232; bh=TJpR6oruumCYfPwA3xSw7LppxUymQtsaKp7hkKBlGFc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=nBrBMnki5oqM8QHDRmJ0X4If0JzOgh58/LDVt5VX3IDaejkHWgFxTD00/R3W2NlR/ t1qwo9bKbQyroQEgHl4MM+4kC1N+C8bJjLXeiLkNd5KCZrceJqPNIk+vYTOn42F3As L+zWeLKhYEys38krZi9zCsXheEI1tLdxSicQcmtY= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Hugh Dickins , Will Deacon , Kiryl Shutsemau , David Hildenbrand , "Aneesh Kumar K.V" , Axel Rasmussen , Chris Li , Christoph Hellwig , Jason Gunthorpe , Johannes Weiner , John Hubbard , Keir Fraser , Konstantin Khlebnikov , Li Zhe , "Matthew Wilcox (Oracle)" , Peter Xu , Rik van Riel , Shivank Garg , Vlastimil Babka , Wei Xu , yangge , Yuanchu Xie , Yu Zhao , Andrew Morton Subject: [PATCH 6.16 058/149] mm/gup: check ref_count instead of lru before migration Date: Mon, 22 Sep 2025 21:29:18 +0200 Message-ID: <20250922192414.337078070@linuxfoundation.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20250922192412.885919229@linuxfoundation.org> References: <20250922192412.885919229@linuxfoundation.org> User-Agent: quilt/0.68 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.16-stable review patch. If anyone has any objections, please let me know. ------------------ From: Hugh Dickins commit 98c6d259319ecf6e8d027abd3f14b81324b8c0ad upstream. Patch series "mm: better GUP pin lru_add_drain_all()", v2. Series of lru_add_drain_all()-related patches, arising from recent mm/gup migration report from Will Deacon. This patch (of 5): Will Deacon reports:- When taking a longterm GUP pin via pin_user_pages(), __gup_longterm_locked() tries to migrate target folios that should not be longterm pinned, for example because they reside in a CMA region or movable zone. This is done by first pinning all of the target folios anyway, collecting all of the longterm-unpinnable target folios into a list, dropping the pins that were just taken and finally handing the list off to migrate_pages() for the actual migration. It is critically important that no unexpected references are held on the folios being migrated, otherwise the migration will fail and pin_user_pages() will return -ENOMEM to its caller. Unfortunately, it is relatively easy to observe migration failures when running pKVM (which uses pin_user_pages() on crosvm's virtual address space to resolve stage-2 page faults from the guest) on a 6.15-based Pixel 6 device and this results in the VM terminating prematurely. In the failure case, 'crosvm' has called mlock(MLOCK_ONFAULT) on its mapping of guest memory prior to the pinning. Subsequently, when pin_user_pages() walks the page-table, the relevant 'pte' is not present and so the faulting logic allocates a new folio, mlocks it with mlock_folio() and maps it in the page-table. Since commit 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page() batch by pagevec"), mlock/munlock operations on a folio (formerly page), are deferred. For example, mlock_folio() takes an additional reference on the target folio before placing it into a per-cpu 'folio_batch' for later processing by mlock_folio_batch(), which drops the refcount once the operation is complete. Processing of the batches is coupled with the LRU batch logic and can be forcefully drained with lru_add_drain_all() but as long as a folio remains unprocessed on the batch, its refcount will be elevated. This deferred batching therefore interacts poorly with the pKVM pinning scenario as we can find ourselves in a situation where the migration code fails to migrate a folio due to the elevated refcount from the pending mlock operation. Hugh Dickins adds:- !folio_test_lru() has never been a very reliable way to tell if an lru_add_drain_all() is worth calling, to remove LRU cache references to make the folio migratable: the LRU flag may be set even while the folio is held with an extra reference in a per-CPU LRU cache. 5.18 commit 2fbb0c10d1e8 may have made it more unreliable. Then 6.11 commit 33dfe9204f29 ("mm/gup: clear the LRU flag of a page before adding to LRU batch") tried to make it reliable, by moving LRU flag clearing; but missed the mlock/munlock batches, so still unreliable as reported. And it turns out to be difficult to extend 33dfe9204f29's LRU flag clearing to the mlock/munlock batches: if they do benefit from batching, mlock/munlock cannot be so effective when easily suppressed while !LRU. Instead, switch to an expected ref_count check, which was more reliable all along: some more false positives (unhelpful drains) than before, and never a guarantee that the folio will prove migratable, but better. Note on PG_private_2: ceph and nfs are still using the deprecated PG_private_2 flag, with the aid of netfs and filemap support functions. Although it is consistently matched by an increment of folio ref_count, folio_expected_ref_count() intentionally does not recognize it, and ceph folio migration currently depends on that for PG_private_2 folios to be rejected. New references to the deprecated flag are discouraged, so do not add it into the collect_longterm_unpinnable_folios() calculation: but longterm pinning of transiently PG_private_2 ceph and nfs folios (an uncommon case) may invoke a redundant lru_add_drain_all(). And this makes easy the backport to earlier releases: up to and including 6.12, btrfs also used PG_private_2, but without a ref_count increment. Note for stable backports: requires 6.16 commit 86ebd50224c0 ("mm: add folio_expected_ref_count() for reference count calculation"). Link: https://lkml.kernel.org/r/41395944-b0e3-c3ac-d648-8ddd70451d28@google.com Link: https://lkml.kernel.org/r/bd1f314a-fca1-8f19-cac0-b936c9614557@google.com Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region") Signed-off-by: Hugh Dickins Reported-by: Will Deacon Closes: https://lore.kernel.org/linux-mm/20250815101858.24352-1-will@kernel.org/ Acked-by: Kiryl Shutsemau Acked-by: David Hildenbrand Cc: "Aneesh Kumar K.V" Cc: Axel Rasmussen Cc: Chris Li Cc: Christoph Hellwig Cc: Jason Gunthorpe Cc: Johannes Weiner Cc: John Hubbard Cc: Keir Fraser Cc: Konstantin Khlebnikov Cc: Li Zhe Cc: Matthew Wilcox (Oracle) Cc: Peter Xu Cc: Rik van Riel Cc: Shivank Garg Cc: Vlastimil Babka Cc: Wei Xu Cc: yangge Cc: Yuanchu Xie Cc: Yu Zhao Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- mm/gup.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/mm/gup.c +++ b/mm/gup.c @@ -2331,7 +2331,8 @@ static unsigned long collect_longterm_un continue; } - if (!folio_test_lru(folio) && drain_allow) { + if (drain_allow && folio_ref_count(folio) != + folio_expected_ref_count(folio) + 1) { lru_add_drain_all(); drain_allow = false; }