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charset=UTF-8 Date: Wed, 19 Aug 2026 11:31:27 -0400 Message-Id: Subject: Re: [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) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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; 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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