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b=f+ffgoSf8IeDZ1MIDekX5uW8oRdMi/p8L9ZVPaGBdADTgB+ZTDflmQE1rwZpfZNzV bhgxD7DB/fqfGhPalA83QT1KMUxvwQQmzaWanpHenakkoGeEe3NRxGz6xaqNX6PhNb WAZeHG9ovL8I8UWwNoKc0fcHbxgCz9dp3/ilWOg84oi6mQDVc4m9hT3weBJeTSvAFt MxEDZcYbh2EGlQ+qSVH+NiUX9rYJx/TAqjseGpBTWCiigKF1bGbHAaAOK1yTjqrKYT hZ5wKhkXpDJftyqEZ78LM6NAEX8b53b8G0xLRf+JYxwH1lx0+HK2ZZoPGGec1aIEK9 Moh2bzIbWwoJg== Message-ID: <54d45248-1a06-43db-b71b-3d5e0fda9de4@kernel.org> Date: Wed, 19 Aug 2026 16:13:02 +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 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> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: ipwae5bkjmomoqoomzd7aag7igpkiq4a X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 5CE54A0006 X-HE-Tag: 1787148792-455040 X-HE-Meta: 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 A2iLSeyB LYltoXArqoUeUsCpXZh3shr+q3wkW88YB+HcP5do1HLV+xwle/5F7QMtS/G7U3VRg8df4SfGI8vApH9mmQh0W9zxOqcEJpYCfohU0dQv5LjTebeWP4usJilPcL0iL5ExQjMO6gopr8132SHECiVTkUSDLYl/14gOYmMGyc3HGZZDpx1Bt02qkGIisuw48rMHp9doWM2Fgdh9Ped04A+zyxm9My1UUefzjCvrFkE+kIML7lBdC3EJUIiJbboTViiKzO5jHB4AVHYbKhVmZfsufB35aoz2uThA+SKrPsaiS/5hEj4MjDxFm1E7w0sMVflPH4nziVzn2bdLC7CA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? -- Cheers, David