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Thu, 23 Oct 2025 05:32:53 -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: [PATCHv2 0/2] Fix SIGBUS semantics with large folios Date: Thu, 23 Oct 2025 10:32:49 +0100 Message-ID: <20251023093251.54146-1-kirill@shutemov.name> X-Mailer: git-send-email 2.50.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 104A840004 X-Stat-Signature: zcopyxskn3sn97egm8um34yd1wt1sqsx X-HE-Tag: 1761211977-794862 X-HE-Meta: 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 mdaS673d 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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: From: Kiryl Shutsemau Accessing memory within a VMA, but beyond i_size rounded up to the next page size, is supposed to generate SIGBUS. Darrick reported[1] an xfstests regression in v6.18-rc1. generic/749 failed due to missing SIGBUS. This was caused by my recent changes that try to fault in the whole folio where possible: 19773df031bc ("mm/fault: try to map the entire file folio in finish_fault()") 357b92761d94 ("mm/filemap: map entire large folio faultaround") These changes did not consider i_size when setting up PTEs, leading to xfstest breakage. However, the problem has been present in the kernel for a long time - since huge tmpfs was introduced in 2016. The kernel happily maps PMD-sized folios as PMD without checking i_size. And huge=always tmpfs allocates PMD-size folios on any writes. I considered this corner case when I implemented a large tmpfs, and my conclusion was that no one in their right mind should rely on receiving a SIGBUS signal when accessing beyond i_size. I cannot imagine how it could be useful for the workload. But apparently filesystem folks care a lot about preserving strict SIGBUS semantics. Generic/749 was introduced last year with reference to POSIX, but no real workloads were mentioned. It also acknowledged the tmpfs deviation from the test case. POSIX indeed says[3]: References within the address range starting at pa and continuing for len bytes to whole pages following the end of an object shall result in delivery of a SIGBUS signal. The patchset fixes the regression introduced by recent changes as well as more subtle SIGBUS breakage due to split failure on truncation. v2: - Fix try_to_unmap() flags; - Add warning if try_to_unmap() fails to unmap the folio; - Adjust comments and commit messages; - Whitespace fixes; v1: - Drop RFC; - Add Signed-off-bys; [1] https://lore.kernel.org/all/20251014175214.GW6188@frogsfrogsfrogs [2] https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/commit/tests/generic/749?h=for-next&id=e4a6b119e5 229599eac96235fb7e683b8a8bdc53 [3] https://pubs.opengroup.org/onlinepubs/9799919799/ Kiryl Shutsemau (2): mm/memory: Do not populate page table entries beyond i_size mm/truncate: Unmap large folio on split failure mm/filemap.c | 18 ++++++++++-------- mm/memory.c | 13 +++++++++++-- mm/truncate.c | 31 +++++++++++++++++++++++++------ 3 files changed, 46 insertions(+), 16 deletions(-) -- 2.50.1