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[130.117.225.111]) by mx.google.com with ESMTPS id ffacd0b85a97d-48e3ece9b53si26733538f8f.209.2026.10.28.07.18.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Oct 2026 07:18:02 -0700 (PDT) Received-SPF: pass (google.com: domain of khorenko@virtuozzo.com designates 130.117.225.111 as permitted sender) client-ip=130.117.225.111; Authentication-Results: mx.google.com; dkim=pass header.i=@virtuozzo.com header.s=relay header.b=GMXb6mvA; spf=pass (google.com: domain of khorenko@virtuozzo.com designates 130.117.225.111 as permitted sender) smtp.mailfrom=khorenko@virtuozzo.com; dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=virtuozzo.com DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=virtuozzo.com; s=relay; h=Content-Type:MIME-Version:Message-Id:Date:Subject :From; bh=h6EXkcvY1iKg18yJdNsV6NEG9eW0lmnhk/STJqnWOjE=; b=GMXb6mvAKBZXJ+30uZS r+3eADFbwE1BD0z+f6qGcznPgJauwPkxoB6/PsEwvAXe/Yc+/FZ0XkRDJLs8jc5Jky88mB3Xn2Ag1 KIAfRqT5LHfa12jH3b+qWBCbzjZW0yCPRikdhF/r82rxd7XcdfnW/FWW5K0SgIYudiZAgdq7gdQMk 0L0Lr+c8Ox2juiInQHDV0pVrZ0qGq65k1uHOqAp6fyxMJX06+g0QINMgYKSxUBp2Tg9IYqLjq9qQU ibSebDbmm6+pWTaQ53yJ5sFeZYyyoQnmnEweBniIO1GmCUbMUdMipN77rscipPzIJFZH17jRbfR62 hWK4W9kqtQU/Vtw==; Received: from ch-demo-asa.virtuozzo.com ([130.117.225.8] helo=f0.sw.ru) by relay.virtuozzo.com with esmtp (Exim 4.96) (envelope-from ) id 1xM4Pn-009nYa-1z; Wed, 28 Oct 2026 15:18:01 +0100 From: Konstantin Khorenko Date: Wed, 28 Oct 2026 15:17:57 +0100 Message-Id: <20261028-b4-prep-khorenko-vstor-151607-numa-migrate-order-v2-0-b973a82a5b0c@virtuozzo.com> MIME-Version: 1.0 X-B4-Tracking: v=1; b=H4sIABUE4moC/52PQXKDMAxFr5LxuvJYBgztqvfoZGGMGrsZcEYGT 0qGu8eQG2Snr6+vJz1EIg6UxNfpIZhySCFOReiPk3DeTheCMBQttNIGle6gr+HGdIOrj0zTNUJ Oc2TABo1qYVpGC2O4sJ0JIg/EUFNljalt9znUoqwt6d9wP5A/wrsAnURxfhlp6f/IzTtwH+1tI ujZTs7vrVcF7FGBkailAo0oGy1Rai3vMq/FiXndsz7sd/0fj2U8YO//kBEKy3RVZRptTNt+58D zEtc1ShdHcd627QmIeo+XSwEAAA== X-Change-ID: 20261028-b4-prep-khorenko-vstor-151607-numa-migrate-order-4e3a664a89d4 To: Pavel Tikhomirov X-Mailer: b4 0.16.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=7892; i=khorenko@virtuozzo.com; h=from:subject:message-id; bh=YxMhxHr9cNUbbDUxP6DJg25Wwl1XJW60vAr5r2Xpc0o=; b=owEBbQKS/ZANAwAKAVGWCkf5YVwqAcsmYgBq4gQYuyFsMYV8RtfJVubQTly1igMTyqz9EkGg4 IoZG8GATmyJAjMEAAEKAB0WIQRWD23IPcXT0GO9CCxRlgpH+WFcKgUCauIEGAAKCRBRlgpH+WFc Kj15D/kBpdk1IJx+PxgrKD85COSPezNQKjbe+fpcJiE7xMB4hgPpwYlpisOtcmIK5qSBBqtV3Pb ldmJkPU/SIIlFQlVN17CyM+QwF+hTFrzCcUK5COhpYaP+rDh2yTAwFh/dEG10XdYoiDxbzT89u5 tld+c6j3tw6TdWkCdFNhwe0I22aWZWMd1m/GE3Pw72RYYjlMCJT7jsti6wf5JMBq83A1pYozQEz qV9BHGUOm03x+muuTJpjLYFP3x+Hp9+s/RcvSt7UOzZDFRjJvpm+x4+Cm4q+vRYk0bQAtkMYTb3 1ZpL098of4jhlrNJm5E0D3BfdTgy4FWKYLZVE6suJ01Mw6EAg+bF9VIUtsHUD4Cr7poNNYuhnNh VoE27QtQNo8C2WB3NjIiFEmbIoE9wJgsoJbw0pA1w/9l1e4s1LQUtknCcJOJblekJnSJwB+tA3y 2m0oCmj0qz9BSuYfnXl54mZb3QQaZt4zHBEAssiQdd37Ms9KeNRUyu2m2bD/AvCSweOXwy65CgX Vg3qEU/9eUkdQBjj3dqA7oPLmtBR26lADxJ0ziKHVZuD94ePciPti+7rScbjd1LCyBV/okbrpe7 tAYbgtnYspjaf4059UCvSMbhWedeCbp6bdsv7OyOZIj9ZK8C41f1L7hVmR2wVmK08cDOqdi7Z56 kpxc1uO81e4R9Ug== X-Developer-Key: i=khorenko@virtuozzo.com; a=openpgp; fpr=560F6DC83DC5D3D063BD082C51960A47F9615C2A X-OZ-Fwd: true Cc: devel@openvz.org Subject: [Devel] [PATCH hci-8.1 v2 0/2] mm/memcontrol: numa_migrate destinations of the source folio order X-BeenThere: devel@openvz.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: OpenVZ development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: devel-bounces@openvz.org Errors-To: devel-bounces@openvz.org memory.numa_migrate, our memcg file that vcmmd uses to move a cgroup's memory between NUMA nodes, allocates a 2 MB destination for every large folio it migrates. Page cache folios of a smaller order get a destination of the wrong size, and the page cache falls apart around it. Patch 1 is the fix, patch 2 a selftest that catches it without crashing the node. What the customer hit ===================== A two-node VMware host runs VMs with vcmmd's NUMA policy enabled, so every few minutes vcmmd writes memory.numa_migrate for a VM's cgroup. qemu writes its backup scratch file on NFS, and NFS sizes each page cache folio to the write, so that file is full of order-1 to order-5 folios. Closing the scratch file dies in nfs4_evict_inode(): kernel BUG at fs/inode.c:737! RIP: 0010:clear_inode+0x75/0x80 mapping->nrpages of the inode is negative, and the vmcore holds order-9 folios of that file with an index not aligned to 512, the refcount of a small folio, and whole xarray ranges of live folios that are no longer in the tree. Reads of the migrated ranges return garbage, so the backups taken on that host while the policy was active should be verified. The same crash was filed as VSTOR-151676. Where the 2 MB comes from ========================= memcg_numa_migrate_new_folio() is the migrate_pages() allocation callback. Its large folio branch dates from vz8 and copies the 4.18 new_page() in mm/mempolicy.c: if (thp_migration_supported() && PageTransHuge(page)) alloc ... HPAGE_PMD_ORDER A compound page on the LRU could only be a PMD-sized THP then, so the test implied the order. Upstream replaced that with folio_order() of the source when its allocators moved to folios, ffe06786b540 ("mm/migrate: Use a folio in alloc_migration_target()"). Our copy kept the shape, only PageTransHuge() became folio_test_large(), which is folio_test_head() and is true for any order from 1 up. NFS and XFS large folios, shmem and anonymous mTHP now all reach it. Why an oversized destination breaks the page cache ================================================== The two sides of the page cache size the folio differently. On insertion __folio_migrate_mapping() takes everything from the source: nr = folio_nr_pages(old), the xarray state at the old index with no xas_set_order(), newfolio->index = old->index, folio_ref_add(newfolio, nr), and xas_store() replaces exactly the slots the source occupied. folio_copy() copies the source pages only. So a 2 MB folio enters the cache as, say, a 4-page one: 4 xarray slots, 4 extra references, nrpages unchanged, 4 pages of valid data. Everything afterwards asks the folio itself, and it answers order 9: - page_cache_delete(): xas_set_order(index, 9) and nrpages -= 512. 512 slots wiped instead of 4, with the index rounded down to 512 so that other live folios lose their slots, and 512 subtracted instead of 4. That is the negative nrpages and the orphaned folios. - __remove_mapping(): expects 1 + 512 references and finds 1 + 4. Reclaim can never free the folio. - filemap_free_folio(): drops 512 references from a handful at eviction. The refcount goes negative and the folio leaks. - reads: folio_file_page() picks the page by its offset inside the folio, but only the first pages were copied. While the folio sits in the cache and is read through its own slots nothing is visible. It breaks the first time something applies folio_nr_pages() or folio_order() to it: invalidation, truncate, reclaim, inode eviction. On NFS cache invalidation is routine (an attribute change), so the bug shows up there instead of staying a silent 2 MB leak per folio. In short: inserted by the source size, removed by the destination size. That only works when the two match, which for a real THP they always did. A debug kernel says the same on the first migration, in mem_cgroup_migrate(): VM_BUG_ON_FOLIO(folio_nr_pages(old) != folio_nr_pages(new)) The patches =========== Patch 1 allocates the destination with folio_order() of the source, GFP_TRANSHUGE_LIGHT for large folios as before. The thp_migration_supported() test goes: migrate_pages() splits PMD-mappable folios itself when THP migration is unsupported, and every other large folio still needs a matching destination. Patch 2 adds tools/testing/selftests/ve/ve_numa_migrate_test.c. It binds a cgroup to one NUMA node, writes a file on a loop-mounted XFS image in 64K chunks so that the page cache holds large folios, migrates the cgroup to the other node through memory.numa_migrate and compares what reads back. memory.current cannot serve as the detector: mem_cgroup_migrate() only transfers the charge, whatever the size mismatch. On an unfixed kernel the comparison fails and the test leaves the image mounted on purpose, since evicting the inode would hit the BUG_ON. The folio sizes are not taken on faith: before the migration the test reads the compound flags of the file's pages through /proc/self/pagemap and /proc/kpageflags, and a file without a large folio fails the test. That is deliberate: a skip would hide that the test no longer reaches the bug, a failure says the setup needs another way to get large folios. After the comparison the test polls memory.numa_stat for a few seconds to see that something was migrated, as memcg flushes its per-cpu counters lazily. It needs root, two NUMA nodes with memory (numa=fake=2 is enough), cgroup2 with memory and cpuset, and mkfs.xfs; it skips otherwise. Testing ======= On a VM booted with numa=fake=2: - the reproducer (64K and 8K writes on XFS from a node-bound cgroup, memory.numa_migrate to the other node, unlink) kills the unfixed debug kernel on the VM_BUG_ON above and the release kernel in clear_inode(); with patch 1 migration succeeds and unlink and eviction complete; - the selftest passes on the fixed kernel and fails on the comparison on 22.11 without taking the node down. Other branches ============== The same patch applies as is to the 20.x branch (the customer's kernel) and is on pr/khorenko-VSTOR-151607-numa-migrate-order-20.x; the function is a plain callback, so it is a ReadyKernel candidate. vz9 carries the same PageTransHuge() code, but 5.14 has no non-PMD large folios on the LRU, so it is not reachable there. https://virtuozzo.atlassian.net/browse/VSTOR-151607 Assisted-by: LLM Signed-off-by: Konstantin Khorenko --- Changes in v2: - Patch 1: the commit message says why the thp_migration_supported() check goes (Pavel). - Patch 2: the folio sizes are checked through /proc/self/pagemap and /proc/kpageflags before the migration; a file without a large folio fails the test on purpose (Pavel). - Patch 2: the read-back comparison runs first and the memory.numa_stat check after it, polled for a few seconds, since memory_numa_stat_show() flushes the per-cpu counters only past a per-CPU threshold (Vasileios). - Reviewed-by from Pavel and Vasileios on both patches. - Link to v1: http://lore.virtuozzo.com/20261028-b4-prep-khorenko-vstor-151607-numa-migrate-order-v1-0-268336526677@virtuozzo.com --- Konstantin Khorenko (2): mm/memcontrol: allocate numa_migrate targets of the source folio order selftests/ve: check memory.numa_migrate keeps the folio order mm/memcontrol.c | 11 +- tools/testing/selftests/ve/Makefile | 1 + tools/testing/selftests/ve/ve_numa_migrate_test.c | 384 ++++++++++++++++++++++ 3 files changed, 391 insertions(+), 5 deletions(-) --- base-commit: b499efa17aa9fb8f20512d1831d1cb61b22e60d3 change-id: 20261028-b4-prep-khorenko-vstor-151607-numa-migrate-order-4e3a664a89d4 Best regards, -- Konstantin Khorenko, Virtuozzo Linux Kernel Team _______________________________________________ Devel mailing list Devel@openvz.org https://lists.openvz.org/mailman/listinfo/devel