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b=HOJKhE6QS17uYsGjRHWX/10MHY1v1FQtfamHsWROPjC4W1e30OS75ZZhE/FlHuLHk JlA6cbf5GrzAQq1aJE2kXvjz/M3mxFgzeH6hcZy1bg887ih+kWUIH3WEvniS1FUHko f8YGIOhYiEMOuzBgg/da9wIR0wfdVGX9uRtn6A43EsvkQfSSfF9AsfZi03KJ6qA9AP V57RYR+ColbGaPdAWY8sGcIL2XVw1JMdpD2Ue1AuGazkqrgvaG7I6vI4WWh6cmRMvx WUd8LeDNIpmMpgdgOFTk9eXWlfvlOTF6HHD8ezwbFSHAScUReKcXCNxQ1cAR83ZCN1 iiub28M0gMLHQ== Date: Wed, 19 Aug 2026 17:09:13 +0100 From: "Lorenzo Stoakes (ARM)" To: Usama Arif Cc: Andrew Morton , baohua@kernel.org, baolin.wang@linux.alibaba.com, david@kernel.org, dev.jain@arm.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.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, stable@vger.kernel.org Subject: Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap Message-ID: References: <20260819101222.3732660-1-usama.arif@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260819101222.3732660-1-usama.arif@linux.dev> X-Stat-Signature: urhmo9x1ykdbp9zp9ayueqaf5r316hhp X-Rspamd-Queue-Id: 8C5FD40005 X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1787155760-569584 X-HE-Meta: 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 yQJKwwZc EPrTzrf1Wnm3IXTlOlWdvRm2BeZklMbaGJMsCkcg8xOJGFpVu5LB96DKcPH/H/re7XxOITAjutRVyHCvAE5Q6PJoNPOJwbluZNJ0Qb8CY4ko5x/l/Id1wE/x2rBLnzdLMOVNI545ZX8GHH5JTKDL6HWGuKRdR2/lLvJaAw4mc1M7CC53YWhocsANG36mgfMrchyIS687Of9uFg09NOs9K07iahQqofp5GEdxsJYJ1uOEsKF+Zz2oYREYboiURG1PKPhSATPqgC0yhf6JPIPv7LaZFNMWuFgs2eF55iSeJOAslBF4MNYMS7x4jNzrV2FWKIkdbSI9HZVMlsKI= 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 03:12:22AM -0700, 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. Good lord. > > 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 Thanks for this! LGTM so: Reviewed-by: Lorenzo Stoakes (ARM) > --- > 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); > if (is_present && pmd_young(pmdval) && > likely(vma_has_recency(vma))) > folio_mark_accessed(folio); > -- > 2.53.0-Meta > -- Cheers, Lorenzo