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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: AEC97140002 X-Stat-Signature: 8yi1nz7dpes6wurrqi7f5srjy886iogy X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1744577114-31578 X-HE-Meta: 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 Da1Npn7h 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 HBiH74hj 1tpUcSIsV1gDTQOrimk6VuL4Qmt9+yyitH9rnIgFxvCvvbthkWJT0AwYQuedc4o62L7A//3jsE4= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 13.04.25 22:20, David Hildenbrand wrote: > On 12.04.25 20:46, syzbot wrote: >> Hello, >> >> syzbot found the following issue on: > > Related to my recent changes > >> >> HEAD commit: 0af2f6be1b42 Linux 6.15-rc1 >> git tree: upstream >> console output: https://syzkaller.appspot.com/x/log.txt?x=1766323f980000 >> kernel config: https://syzkaller.appspot.com/x/.config?x=f175b153b655dbb3 > > CONFIG_ARCH_WANTS_THP_SWAP=y > CONFIG_MM_ID=y > CONFIG_TRANSPARENT_HUGEPAGE=y > # CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS is not set > CONFIG_TRANSPARENT_HUGEPAGE_MADVISE=y > # CONFIG_TRANSPARENT_HUGEPAGE_NEVER is not set > CONFIG_THP_SWAP=y > CONFIG_READ_ONLY_THP_FOR_FS=y > # CONFIG_NO_PAGE_MAPCOUNT is not set > CONFIG_PAGE_MAPCOUNT=y > >> dashboard link: https://syzkaller.appspot.com/bug?extid=5e8feb543ca8e12e0ede >> compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40 >> >> Unfortunately, I don't have any reproducer for this issue yet. >> >> Downloadable assets: >> disk image: https://storage.googleapis.com/syzbot-assets/f1d71d1bf77d/disk-0af2f6be.raw.xz >> vmlinux: https://storage.googleapis.com/syzbot-assets/7f1638f065da/vmlinux-0af2f6be.xz >> kernel image: https://storage.googleapis.com/syzbot-assets/9b3e49834705/bzImage-0af2f6be.xz >> >> IMPORTANT: if you fix the issue, please add the following tag to the commit: >> Reported-by: syzbot+5e8feb543ca8e12e0ede@syzkaller.appspotmail.com >> >> ------------[ cut here ]------------ >> WARNING: CPU: 0 PID: 7165 at mm/memory.c:3738 __wp_can_reuse_large_anon_folio mm/memory.c:3738 [inline] > > VM_WARN_ON_ONCE(folio_entire_mapcount(folio)); > > Which is rather unexpected. I know we had a scenario (remapping a THP?) > where we would have a PMD mapping and a PTE mapping of an exclusive anon > folio for a very short time. But, IIRC locking should make sure that > that cannot be observed by some other page table walker. > Ah, it likely is a (harless) race, when process A and process B cow-share a PMD THP, and process A write-faults on a PTE mapping of the THP while process B concurrently unmaps the PMD mapping of the THP. In __folio_remove_rmap(), for RMAP_LEVEL_PMD in case of CONFIG_PAGE_MAPCOUNT=y, we'll do folio_dec_large_mapcount(folio, vma); last = atomic_add_negative(-1, &folio->_entire_mapcount); So after decrementing the large mapcount, the folio will be indicated as "exclusive" to process A. Process B, still has to decrement the entire mapcount, but process A might already run into the entire_mapcount sanity check. In do_wp_page(), we'd later fail the "folio_large_mapcount(folio) != folio_ref_count(folio)" test until process B is completely done with unmapping the folio. Maybe we should just move these sanity checks after the refcount check, or reverse the mapcount decrement order. I'll think about that. -- Cheers, David / dhildenb