Patch series "Fix SIGBUS semantics with large folios", v3. 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. This patch (of 2): Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are supposed to generate SIGBUS. Recent changes attempted to fault in full folio where possible. They did not respect i_size, which led to populating PTEs beyond i_size and breaking SIGBUS semantics. Darrick reported generic/749 breakage because of this. However, the problem existed before the recent changes. With huge=always tmpfs, any write to a file leads to PMD-size allocation. Following the fault-in of the folio will install PMD mapping regardless of i_size. Fix filemap_map_pages() and finish_fault() to not install: - PTEs beyond i_size; - PMD mappings across i_size; Make an exception for shmem/tmpfs that for long time intentionally mapped with PMDs across i_size. Link: https://lkml.kernel.org/r/20251027115636.82382-1-kirill@shutemov.name Link: https://lkml.kernel.org/r/20251027115636.82382-2-kirill@shutemov.name Signed-off-by: Kiryl Shutsemau <kas@kernel.org> Fixes: 6795801366da ("xfs: Support large folios") Reported-by: "Darrick J. Wong" <djwong@kernel.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Baolin Wang <baolin.wang@linux.alibaba.com> Cc: Christian Brauner <brauner@kernel.org> Cc: Dave Chinner <david@fromorbit.com> Cc: David Hildenbrand <david@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Liam Howlett <liam.howlett@oracle.com> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> Cc: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Michal Hocko <mhocko@suse.com> Cc: Mike Rapoport <rppt@kernel.org> Cc: Rik van Riel <riel@surriel.com> Cc: Shakeel Butt <shakeel.butt@linux.dev> Cc: Suren Baghdasaryan <surenb@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>