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Received: from fangorn.home.surriel.com ([10.0.13.7]) by shelob.surriel.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.97.1) (envelope-from ) id 1wevH7-000000006qJ-03pK; Wed, 01 Jul 2026 09:46:49 -0400 From: Rik van Riel To: linux-kernel@vger.kernel.org Cc: kernel-team@meta.com, linux-mm@kvack.org, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, yang@os.amperecomputing.com, Rik van Riel Subject: [PATCH] mm/huge_memory: set PG_has_hwpoisoned only after new folio head is established Date: Wed, 1 Jul 2026 09:46:22 -0400 Message-ID: <20260701134622.3152896-1-riel@surriel.com> X-Mailer: git-send-email 2.54.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: C6F8E18000B X-Stat-Signature: wtybn8ox34ionha1qgy9hyhs9r6dnm15 X-HE-Tag: 1782913622-187902 X-HE-Meta: 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 maBhitfB RrdHAUKMBSs1wv6UgNUbsI0bBzfj3DtvICksoH/cn5+PEoxRv07EFtiAEYSeF9TOz+Mj2WT0jNzJ6Ie5QvKpmZAC590Wc+aDOAdYNfi4iemfayN5FGJ4XQ+AHigWU0GC0xgd29SC2pIIJUCdydWpB7+stsXg88GWYqFwYl6B9JU8C4LyFFusTGsiwfFJZNHC0JBLJKHvqjCYWoeZidyDol3+JUhB1KFXLbMrDAl3soMUeD1ycFDkKDikDI506sHWaUe7BXQsD9xAerKHtRDmR/ryjqjpIozFXWkAmNsJj4zHwC5s6IdKSbRPaX+iwov6Am9fN Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: __split_folio_to_order() copies the hwpoison state onto each new sub-folio while splitting a folio to a non-zero order. It did so via if (handle_hwpoison && page_range_has_hwpoisoned(new_head, new_nr_pages)) folio_set_has_hwpoisoned(new_folio); *before* clear_compound_head(new_head)/prep_compound_page(new_head, ...) turn @new_head from a tail page into a proper folio head. PG_has_hwpoisoned is a FOLIO_SECOND_PAGE flag, so folio_set_has_hwpoisoned() resolves to folio_flags(folio, 1). With the new compound_info-based page-flags layout, folio_flags() asserts the page is not a tail: VM_BUG_ON_PGFLAGS(page->compound_info & 1, page); VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags.f), page); At the original call site @new_head still has the tail marker (compound_info bit 0 set, PG_head clear), so on CONFIG_DEBUG_VM kernels this hits: kernel BUG at include/linux/page-flags.h:354 folio_flags+0x82 folio_set_has_hwpoisoned __split_folio_to_order __split_unmapped_folio __folio_split truncate_inode_partial_folio (shmem hole-punch / MADV_REMOVE) Reproduced by syzkaller: hwpoison-inject a few subpages of a large shmem folio, then MADV_REMOVE (fallocate punch hole) on the same range, which splits the partial folio to a non-zero order. Move the folio_set_has_hwpoisoned() call to after clear_compound_head()/prep_compound_page(), where @new_folio is a real order-new_order head folio (handle_hwpoison implies new_order != 0, so a second page always exists). The flag still lands on the same struct page (page[1] of the new folio); only the ordering relative to compound-head setup changes, satisfying the FOLIO_SECOND_PAGE precondition. Signed-off-by: Rik van Riel Assisted-by: Claude:claude-opus-4-8 Fixes: fa5a06170036 ("mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split to >0 order") --- mm/huge_memory.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 2bccb0a53a0a..ee7ecb3b45c6 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3587,10 +3587,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order, (1L << PG_dropbehind) | LRU_GEN_MASK | LRU_REFS_MASK)); - if (handle_hwpoison && - page_range_has_hwpoisoned(new_head, new_nr_pages)) - folio_set_has_hwpoisoned(new_folio); - new_folio->mapping = folio->mapping; new_folio->index = folio->index + i; @@ -3612,6 +3608,18 @@ static void __split_folio_to_order(struct folio *folio, int old_order, folio_set_large_rmappable(new_folio); } + /* + * PG_has_hwpoisoned is a FOLIO_SECOND_PAGE flag, so it can only + * be set once @new_folio is a real (head) folio. Defer setting + * it until after clear_compound_head()/prep_compound_page() have + * turned @new_head from a tail page into a proper folio head; + * otherwise folio_flags() trips on (page->compound_info & 1). + * handle_hwpoison implies new_order != 0. + */ + if (handle_hwpoison && + page_range_has_hwpoisoned(new_head, new_nr_pages)) + folio_set_has_hwpoisoned(new_folio); + if (folio_test_young(folio)) folio_set_young(new_folio); if (folio_test_idle(folio)) -- 2.53.0-Meta