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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>
Linux kernel
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