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 (cherry picked from commit 74207de2ba10c2973334906822dc94d2e859ffc5) Signed-off-by: Kiryl Shutsemau kas@kernel.org --- mm/filemap.c | 20 +++++++++++++++----- mm/memory.c | 24 +++++++++++++++++++++++- 2 files changed, 38 insertions(+), 6 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c index bff0abf4c3a7..3b73e62311ce 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3440,13 +3440,27 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, struct page *page; unsigned int mmap_miss = READ_ONCE(file->f_ra.mmap_miss); vm_fault_t ret = 0; + bool can_map_large;
rcu_read_lock(); folio = first_map_page(mapping, &xas, end_pgoff); if (!folio) goto out;
- if (filemap_map_pmd(vmf, &folio->page)) { + file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1; + end_pgoff = min(end_pgoff, file_end); + + /* + * Do not allow to map with PTEs beyond i_size and with PMD + * across i_size to preserve SIGBUS semantics. + * + * Make an exception for shmem/tmpfs that for long time + * intentionally mapped with PMDs across i_size. + */ + can_map_large = shmem_mapping(mapping) || + file_end >= folio_next_index(folio); + + if (can_map_large && filemap_map_pmd(vmf, &folio->page)) { ret = VM_FAULT_NOPAGE; goto out; } @@ -3454,10 +3468,6 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT); vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
- file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1; - if (end_pgoff > file_end) - end_pgoff = file_end; - do { again: page = folio_file_page(folio, xas.xa_index); diff --git a/mm/memory.c b/mm/memory.c index 454d918449b3..f0b506acfcc5 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -67,6 +67,7 @@ #include <linux/gfp.h> #include <linux/migrate.h> #include <linux/string.h> +#include <linux/shmem_fs.h> #include <linux/memory-tiers.h> #include <linux/debugfs.h> #include <linux/userfaultfd_k.h> @@ -4452,6 +4453,8 @@ static bool vmf_pte_changed(struct vm_fault *vmf) vm_fault_t finish_fault(struct vm_fault *vmf) { struct vm_area_struct *vma = vmf->vma; + bool needs_fallback = false; + struct folio *folio; struct page *page; vm_fault_t ret;
@@ -4461,6 +4464,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf) else page = vmf->page;
+ folio = page_folio(page); + /* * check even for read faults because we might have lost our CoWed * page @@ -4471,8 +4476,25 @@ vm_fault_t finish_fault(struct vm_fault *vmf) return ret; }
+ if (!needs_fallback && vma->vm_file) { + struct address_space *mapping = vma->vm_file->f_mapping; + pgoff_t file_end; + + file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE); + + /* + * Do not allow to map with PTEs beyond i_size and with PMD + * across i_size to preserve SIGBUS semantics. + * + * Make an exception for shmem/tmpfs that for long time + * intentionally mapped with PMDs across i_size. + */ + needs_fallback = !shmem_mapping(mapping) && + file_end < folio_next_index(folio); + } + if (pmd_none(*vmf->pmd)) { - if (PageTransCompound(page)) { + if (!needs_fallback && PageTransCompound(page)) { ret = do_set_pmd(vmf, page); if (ret != VM_FAULT_FALLBACK) return ret;