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Thu, 08 Jan 2026 18:17:23 -0800 (PST) Date: Fri, 9 Jan 2026 10:17:19 +0800 From: Jinchao Wang To: Matthew Wilcox Cc: Muchun Song , Oscar Salvador , David Hildenbrand , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, syzbot+2d9c96466c978346b55f@syzkaller.appspotmail.com, Zi Yan Subject: Re: [PATCH 2/2] Fix an AB-BA deadlock in hugetlbfs_punch_hole() involving page migration. Message-ID: References: <20260108123957.1123502-1-wangjinchao600@gmail.com> <20260108123957.1123502-2-wangjinchao600@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Stat-Signature: arpehpbfwfhchibuxwjztc1ud4aqq17q X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 7800540004 X-HE-Tag: 1767925045-494424 X-HE-Meta: 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 63GFWmGI 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 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 Thu, Jan 08, 2026 at 02:09:19PM +0000, Matthew Wilcox wrote: > On Thu, Jan 08, 2026 at 08:39:25PM +0800, Jinchao Wang wrote: > > The deadlock occurs due to the following lock ordering: > > > > Task A (punch_hole): Task B (migration): > > -------------------- ------------------- > > 1. i_mmap_lock_write(mapping) 1. folio_lock(folio) > > 2. folio_lock(folio) 2. i_mmap_lock_read(mapping) > > (blocks waiting for B) (blocks waiting for A) > > > > Task A is blocked in the punch-hole path: > > hugetlbfs_fallocate > > hugetlbfs_punch_hole > > hugetlbfs_zero_partial_page > > filemap_lock_hugetlb_folio > > filemap_lock_folio > > __filemap_get_folio > > folio_lock > > > > Task B is blocked in the migration path: > > migrate_pages > > migrate_hugetlbs > > unmap_and_move_huge_page > > remove_migration_ptes > > __rmap_walk_file > > i_mmap_lock_read > > > > To break this circular dependency, use filemap_lock_folio_nowait() in > > the punch-hole path. If the folio is already locked, Task A drops the > > i_mmap_rwsem and retries. This allows Task B to finish its rmap walk > > and release the folio lock. > > It looks like you didn't read the lock ordering at the top of mm/rmap.c > carefully enough: > > * hugetlbfs PageHuge() take locks in this order: > * hugetlb_fault_mutex (hugetlbfs specific page fault mutex) > * vma_lock (hugetlb specific lock for pmd_sharing) > * mapping->i_mmap_rwsem (also used for hugetlb pmd sharing) > * folio_lock > Thanks for the correction, Matthew. > So page migration is the one taking locks in the wrong order, not > holepunch. Maybe something like this instead? > I will test your suggested change and resend the fix. > > diff --git a/mm/migrate.c b/mm/migrate.c > index 5169f9717f60..4688b9e38cd2 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1458,6 +1458,7 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio, > int page_was_mapped = 0; > struct anon_vma *anon_vma = NULL; > struct address_space *mapping = NULL; > + enum ttu_flags ttu = 0; > > if (folio_ref_count(src) == 1) { > /* page was freed from under us. So we are done. */ > @@ -1498,8 +1499,6 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio, > goto put_anon; > > if (folio_mapped(src)) { > - enum ttu_flags ttu = 0; > - > if (!folio_test_anon(src)) { > /* > * In shared mappings, try_to_unmap could potentially > @@ -1516,16 +1515,17 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio, > > try_to_migrate(src, ttu); > page_was_mapped = 1; > - > - if (ttu & TTU_RMAP_LOCKED) > - i_mmap_unlock_write(mapping); > } > > if (!folio_mapped(src)) > rc = move_to_new_folio(dst, src, mode); > > if (page_was_mapped) > - remove_migration_ptes(src, !rc ? dst : src, 0); > + remove_migration_ptes(src, !rc ? dst : src, > + ttu ? RMP_LOCKED : 0); > + > + if (ttu & TTU_RMAP_LOCKED) > + i_mmap_unlock_write(mapping); > > unlock_put_anon: > folio_unlock(dst);