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 4A818635; Sat, 13 Sep 2025 20:09:21 +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=1757794162; cv=none; b=P5UkiJ2Pv0ilBYW8k2eZyCeMBqTNukFQ6NDnzOL7ufthFKYoPG/AoEL86LJ9lIat/LJq8+ZQr0YU/vuSMzq1b+AJ4iTiENNMDe06H/KaAqXM3YaUjMziNU3HZrsrVyfuqfcsThmJo+4wyx4IImArBSiy1qWoDgLVS8GEq4pmbus= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757794162; c=relaxed/simple; bh=lYbzApe4nVRI3RP7ZDHFu+pB/5At6W71kgFlLi8fCIg=; h=Date:To:From:Subject:Message-Id; b=DTAOG1W76iywOwZuJCQivnAL14Uezs9f/1zywJB41UHtkPQhh8NA4Y+gvsrbTvQXOTbubbjJ2uWL61h6LLSxciyVR+SRnboZbFIgtQ0yp0TrkudRJWo0Wj6Vjvmwi96pW636crVCgrZZaflbzTyO9/uLfm80ofniOiztIUVqdM4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=BajHE4D7; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="BajHE4D7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C899FC4CEEB; Sat, 13 Sep 2025 20:09:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1757794160; bh=lYbzApe4nVRI3RP7ZDHFu+pB/5At6W71kgFlLi8fCIg=; h=Date:To:From:Subject:From; b=BajHE4D7N5CwkMaGeZNVK9mmYt7SFVsN2Aon2eGoDNr26SWhaO1FCTER2dRjDyFNX 3u5q69iVyk7Lr26yxn3RY2QGpGWyNkLh8A3zq6FL6WQ4k1uukUABbaVep54VyxvTDI E1L2GYcGdpctJuD4ecfio8eKQpCjV5FprrtH4PvU= Date: Sat, 13 Sep 2025 13:09:20 -0700 To: mm-commits@vger.kernel.org,yuzhao@google.com,yuanchu@google.com,yangge1116@126.com,willy@infradead.org,will@kernel.org,weixugc@google.com,vbabka@suse.cz,stable@vger.kernel.org,shivankg@amd.com,riel@surriel.com,peterx@redhat.com,lizhe.67@bytedance.com,koct9i@gmail.com,keirf@google.com,kas@kernel.org,jhubbard@nvidia.com,jgg@ziepe.ca,hch@infradead.org,hannes@cmpxchg.org,david@redhat.com,chrisl@kernel.org,axelrasmussen@google.com,aneesh.kumar@kernel.org,hughd@google.com,akpm@linux-foundation.org From: Andrew Morton Subject: [merged mm-hotfixes-stable] mm-gup-check-ref_count-instead-of-lru-before-migration.patch removed from -mm tree Message-Id: <20250913200920.C899FC4CEEB@smtp.kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The quilt patch titled Subject: mm/gup: check ref_count instead of lru before migration has been removed from the -mm tree. Its filename was mm-gup-check-ref_count-instead-of-lru-before-migration.patch This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm ------------------------------------------------------ From: Hugh Dickins Subject: mm/gup: check ref_count instead of lru before migration Date: Mon, 8 Sep 2025 15:15:03 -0700 (PDT) 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 --- mm/gup.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/mm/gup.c~mm-gup-check-ref_count-instead-of-lru-before-migration +++ a/mm/gup.c @@ -2307,7 +2307,8 @@ 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; } _ Patches currently in -mm which might be from hughd@google.com are mm-lru_add_drain_all-do-local-lru_add_drain-first.patch