The patch titled Subject: mm/truncate: unmap large folio on split failure has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-truncate-unmap-large-folio-on-split-failure.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Kiryl Shutsemau kas@kernel.org Subject: mm/truncate: unmap large folio on split failure Date: Mon, 27 Oct 2025 11:56:36 +0000

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.

Link: https://lkml.kernel.org/r/20251027115636.82382-3-kirill@shutemov.name Signed-off-by: Kiryl Shutsemau kas@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: "Darrick J. Wong" djwong@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 ---

mm/truncate.c | 35 +++++++++++++++++++++++++++++------ 1 file changed, 29 insertions(+), 6 deletions(-)

--- a/mm/truncate.c~mm-truncate-unmap-large-folio-on-split-failure +++ a/mm/truncate.c @@ -177,6 +177,32 @@ int truncate_inode_folio(struct address_ 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

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 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: _

Patches currently in -mm which might be from kas@kernel.org are

mm-memory-do-not-populate-page-table-entries-beyond-i_size.patch mm-truncate-unmap-large-folio-on-split-failure.patch