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b=n7I8mRiztlyvV8aXq8zcIvHLh0wxIwAIpvk6GPsebAgpC+B6T0Df7rQN1t1WELX2r a4nyQKh2iWEN3pREWaFH4b9ED1H6Ht02QEnWDsZwaOswqv5w1sKwJ0nMFvwdgZsVz6 gxWmK8QnxknJlysgG5ab3tUnrF1uk7e5+c1j65v/fc3uxqu8+5OyLbyF9JMpql36eL dnzq3oSCYmzCi/SIAqyzfTLItMNuOUpj1m6cZIO5cQSwSdcy536jNfyZNsLid4FTGk he8/ceCooMLT9bT1L7EtGCzCMAbYnAAyw3xIdFkceD60o4jQcIcrAfQ/79BgFB+h3D c5Ki05VjckAXg== Message-ID: Date: Wed, 19 Aug 2026 16:13:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap From: "David Hildenbrand (Arm)" To: Usama Arif , Andrew Morton , baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, nico.pache@linux.dev, ryan.roberts@arm.com, ziy@nvidia.com, kas@kernel.org, nphamcs@gmail.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com Cc: stable@vger.kernel.org References: <20260819101222.3732660-1-usama.arif@linux.dev> <54d45248-1a06-43db-b71b-3d5e0fda9de4@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: cpz9d899nzq1i4n9r3zwcu7m7ajgpjq1 X-Rspamd-Queue-Id: DD4098000A X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1787148833-594227 X-HE-Meta: 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 Pb8YZQAE xv5IWoISBIHqAvHz7CQU/vFJOxQCQLk4ClVT6sdcdx/o2mIccsBx33YSt4wFx+VAI/tJTSb+aBr6VaJTvqg2K1bGe/aaxhtp6pZ5BcWEsnG0pdTiHQs+ceaa/2O46g9mNKXgIWnXaDy+OiDcW8SrrZyqAXnNf+knZfc7E81/X//lQoCG+/Mf4Pt/v0gniAfgAY3pp2zu3F9K/Bzn2YJJL7ter5ff6YC/m44/PB1s3ZEN27tzsbQGhPGqFUO8fPyIVcqb0ttT2rl9mUHGVSmJ9cCzFUQaC2hlDUsWaVGnD4Y3FomVoOpYGqjLq6AHikvp9tzHU/PvVf3pZOsI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/19/26 16:13, David Hildenbrand (Arm) wrote: > On 8/19/26 12:12, 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=within_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=never, 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(+) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index ced400f72d43a..afbb5974bd225 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -2449,6 +2449,8 @@ static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma, >> add_mm_counter(mm, mm_counter_file(folio), >> -HPAGE_PMD_NR); >> >> + if (is_present && pmd_dirty(pmdval)) >> + folio_mark_dirty(folio); > > We don't need that for anon folios, though. So best to just resemble > zap_present_folio_ptes() logic and do it only for !anon folios? > I'm stupid, that check is not visible in the diff above :) Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David