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Mon, 27 Oct 2025 07:56:50 -0400 (EDT) From: Kiryl Shutsemau To: Andrew Morton , David Hildenbrand , Hugh Dickins , Matthew Wilcox , Alexander Viro , Christian Brauner Cc: Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Rik van Riel , Harry Yoo , Johannes Weiner , Shakeel Butt , Baolin Wang , "Darrick J. Wong" , Dave Chinner , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv3 2/2] mm/truncate: Unmap large folio on split failure Date: Mon, 27 Oct 2025 11:56:36 +0000 Message-ID: <20251027115636.82382-3-kirill@shutemov.name> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20251027115636.82382-1-kirill@shutemov.name> References: <20251027115636.82382-1-kirill@shutemov.name> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: A89FA120002 X-Rspamd-Server: rspam02 X-Stat-Signature: ug16unyzed6uxdho8z83jriehff4u8gc X-HE-Tag: 1761566211-190223 X-HE-Meta: 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 I+DKillY FVxnt1kdZAyaoplF5l3PBCWMxLnR8z6+NRo6+odYX8hq+SBVt+tRl8NBlEkpQXQVSoa7SQU13iLUARUSJgEjpaDNJEUea0iefT4Rsjnivh9Pw1+s23YvPyvIo5z/MxxyrhDVR6AVdFzhL2Mrox27/8wHKi8iC6kY+zmptpOwq0ePp+MA2us7pozwnRJ3+Iw0uZ14P5SzAIqfidhGwxERmV/SYXMEzyvnq+jsohh8XUIl/50NbmbyBIZYO2x5RYzJR29GpQDzJPmaA91FbZeHhXyD//fIc/U/i3JP8IBXLzfr9L98b1XYqcCfA4hkc+OzS+CCX1LwSvOUE58vUlgzyfgdsUowqk0trS0vgcRo2IpvFQDzHDGHueBZt4qcSwYhHwXxf3MKqtI6SCPzjviWrHmpE3gpqVcUZ68L3 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: From: Kiryl Shutsemau Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are supposed to generate SIGBUS. This behavior might not be respected on truncation. During truncation, the kernel splits a large folio in order to reclaim memory. As a side effect, it unmaps the folio and destroys PMD mappings of the folio. The folio will be refaulted as PTEs and SIGBUS semantics are preserved. However, if the split fails, PMD mappings are preserved and the user will not receive SIGBUS on any accesses within the PMD. Unmap the folio on split failure. It will lead to refault as PTEs and preserve SIGBUS semantics. Make an exception for shmem/tmpfs that for long time intentionally mapped with PMDs across i_size. Signed-off-by: Kiryl Shutsemau --- mm/truncate.c | 35 +++++++++++++++++++++++++++++------ 1 file changed, 29 insertions(+), 6 deletions(-) diff --git a/mm/truncate.c b/mm/truncate.c index 9210cf808f5c..3c5a50ae3274 100644 --- a/mm/truncate.c +++ b/mm/truncate.c @@ -177,6 +177,32 @@ int truncate_inode_folio(struct address_space *mapping, struct folio *folio) return 0; } +static int try_folio_split_or_unmap(struct folio *folio, struct page *split_at, + unsigned long min_order) +{ + enum ttu_flags ttu_flags = + TTU_SYNC | + TTU_SPLIT_HUGE_PMD | + TTU_IGNORE_MLOCK; + int ret; + + ret = try_folio_split_to_order(folio, split_at, min_order); + + /* + * If the split fails, unmap the folio, so it will be refaulted + * with PTEs to respect SIGBUS semantics. + * + * Make an exception for shmem/tmpfs that for long time + * intentionally mapped with PMDs across i_size. + */ + if (ret && !shmem_mapping(folio->mapping)) { + try_to_unmap(folio, ttu_flags); + WARN_ON(folio_mapped(folio)); + } + + return ret; +} + /* * Handle partial folios. The folio may be entirely within the * range if a split has raced with us. If not, we zero the part of the @@ -226,7 +252,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) min_order = mapping_min_folio_order(folio->mapping); split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE); - if (!try_folio_split_to_order(folio, split_at, min_order)) { + if (!try_folio_split_or_unmap(folio, split_at, min_order)) { /* * try to split at offset + length to make sure folios within * the range can be dropped, especially to avoid memory waste @@ -250,13 +276,10 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) if (!folio_trylock(folio2)) goto out; - /* - * make sure folio2 is large and does not change its mapping. - * Its split result does not matter here. - */ + /* make sure folio2 is large and does not change its mapping */ if (folio_test_large(folio2) && folio2->mapping == folio->mapping) - try_folio_split_to_order(folio2, split_at2, min_order); + try_folio_split_or_unmap(folio2, split_at2, min_order); folio_unlock(folio2); out: -- 2.50.1