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[PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap Cc: To: "Usama Arif" , "Andrew Morton" , , , , , , , , , , , , , , , , , From: "Zi Yan" X-Mailer: aerc 0.21.0 References: <20260819101222.3732660-1-usama.arif@linux.dev> In-Reply-To: <20260819101222.3732660-1-usama.arif@linux.dev> X-ClientProxiedBy: BN9PR03CA0359.namprd03.prod.outlook.com (2603:10b6:408:f6::34) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|BL3PR12MB6522:EE_ X-MS-Office365-Filtering-Correlation-Id: 22f584ee-4b14-462a-ea07-08defe06f039 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|7416014|376014|366016|1800799024|921020|6133799003|56012099006|10067099003|11063799006|22082099003|18002099003; X-Microsoft-Antispam-Message-Info: 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X-Stat-Signature: dpsxjyottfpi7o6968foihq9u9i5xfri X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: A3D4C20007 X-HE-Tag: 1787153494-458395 X-HE-Meta: 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 jiYJG099 Pn6oWKF7csltn9D7H9vP7tBlwWoYAXcHmThjnh5ktcK7be6RY779c78Q8uN+EifMHVE1SRAVhdGXceWZ65y/TRIOo1ERUv8ifZdHg+0UJuH39sifRdcC9XiQ2ylistisbpz+gSDHhsN0824BkLGGQiqJDQtnPo/MCeUgsCqod7pNJJpZ5xe8b6FLVmFGP7jv2pMNM3V9RTWUcwwrdlgqltyF63970UhzAvNc7Lvcf4F92j1UqqDzoeodYc6pTLNqiIFc3QyBk/a7irn6sTXJRhxrw/ftsxX3qg0/WpeFiax3zZtD22Imj06s//yzPbVivEnSdQRHMfALpp9fzPsh7bKhG+hf6H015i/oDUOrMpa0V3fkaAYGqBjzQ/u1oVhHdStPO63e3iKnD4EsoDC9gmmrJytBmdG4CyvsOJGzhGsqliUr/7/r2jwIIuhpAtju4hMtRjGh0/ZAG2zeNi9nUFHxCXg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed Aug 19, 2026 at 6:12 AM EDT, Usama Arif wrote: > zap_huge_pmd_folio() propagates the pmd young bit to the folio for the > file case, but not the dirty bit. The pte path does propagate it, in > zap_present_folio_ptes() and so does the pmd split path, in > __split_huge_pmd_locked(). > > For most file mappings the omission is harmless, because writing to a > shared file mapping goes through page_mkwrite(), which dirties the > folio. tmpfs is different: it has no page_mkwrite(), and > vma_wants_writenotify() is false for it, so a *read* fault on a > MAP_SHARED tmpfs mapping installs a writable pmd via do_read_fault(). > do_read_fault() does not call fault_dirty_shared_page(), so subsequent > stores through that mapping set only the hardware dirty bit in the pmd > and never call folio_mark_dirty(). A shmem folio allocated by a fault > is marked uptodate but not dirty (see the clear: block in > shmem_get_folio_gfp()), so PG_dirty is never set at all. > > Unmapping such a folio - munmap(), or exit_mmap() when the process dies > - then loses the only record that it was written, because zap_huge_pmd() > drops the pmd without transferring the dirty bit. Reclaim afterwards > sees a clean shmem folio: the whole swap-out block in > shrink_folio_list() is inside "if (folio_test_dirty(folio))", so > pageout() is skipped and the folio falls into __remove_mapping(). > There, folio_is_file_lru() is false for a swapbacked folio, so no shadow > entry is created and __filemap_remove_folio(folio, NULL) simply empties > the i_pages slot. The data is freed without ever being written to swap, > and the next fault on that index returns a freshly zeroed folio. > > This is silent data loss for any process that keeps state in a > MAP_SHARED tmpfs segment across an unmap - for example a cache handed > from one process generation to the next through /dev/shm. It requires > the folio to be PMD-mapped, so it only shows up once shmem THP is > enabled (which is what we did in Meta fleet and started noticing crashes)= ; > with THP off the pte path transfers the dirty bit correctly. > It also only becomes visible when swap is enabled, because with no swap > device shmem folios (which are on the anon LRU) are not scanned by > reclaim at all, so the clean folio is never dropped. > > Reproduced on x86_64 with a tmpfs mounted huge=3Dwithin_size: read-fault = a > 2MB-backed region, write a known pattern through the resulting mapping, > munmap, force reclaim of the cgroup, then re-map and read back. Without > this patch the region reads back as zeros and vmstat shows zswpout 0 - > the data was discarded rather than swapped. With this patch the region > reads back correctly and the pages are swapped out as expected. With > huge=3Dnever, or when the first touch is a write, the test passes either > way. > > Fixes: 800d8c63b2e9 ("shmem: add huge pages support") > Cc: > Signed-off-by: Usama Arif > --- > mm/huge_memory.c | 2 ++ > 1 file changed, 2 insertions(+) > Make sense. Thanks. Reviewed-by: Zi Yan --=20 Best Regards, Yan, Zi