From: Usama Arif Date: Mon, 25 May 2026 14:57:51 +0000 (-0700) Subject: mm: make mmap_miss accounting symmetric for VM_SEQ_READ X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=eb4c458a9803c3c75ee27d567a3a2ff0cc66da98;p=thirdparty%2Flinux.git mm: make mmap_miss accounting symmetric for VM_SEQ_READ do_sync_mmap_readahead() skips both the mmap_miss increment and the MMAP_LOTSAMISS check for VM_SEQ_READ mappings, since sequential access is non-speculative and should always read ahead. The two decrement sites in do_async_mmap_readahead() and filemap_map_pages() do not mirror this skip, so concurrent faults on a VM_SEQ_READ mapping can still drive ra->mmap_miss down to zero through the decrement paths even though nothing in the sync path ever increments it. The counter itself is per-file (file->f_ra.mmap_miss), so it can be moved by any VMA mapping the file, not just the one currently faulting. Skip the decrement for VM_SEQ_READ in both decrement sites so the counter only moves for mappings that also participate in the increment side. No functional change for VM_SEQ_READ users, since the increment-side gate already prevents the counter from being consulted on their behalf, but it stops a VM_SEQ_READ mapping from biasing the counter for other mappings of the same file. Link: https://lore.kernel.org/20260525145751.2671248-1-usama.arif@linux.dev Signed-off-by: Usama Arif Closes: https://lore.kernel.org/all/8edc8cd0-f65c-4456-9b3f-362e744c9a96@linux.dev/ Reviewed-by: William Kucharski Reviewed-by: Jan Kara Cc: David Hildenbrand Cc: Johannes Weiner Cc: Matthew Wilcox (Oracle) Cc: Shakeel Butt Signed-off-by: Andrew Morton --- diff --git a/mm/filemap.c b/mm/filemap.c index 4263d9775998..6bf0b540ef19 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3434,8 +3434,13 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, * Don't touch the mmap_miss counter to avoid decreasing it multiple * times for a single folio and break the balance with mmap_miss * increase in do_sync_mmap_readahead(). + * + * VM_SEQ_READ mappings skip the mmap_miss increment in + * do_sync_mmap_readahead(), so skip the decrement here as well to + * keep the counter symmetric. */ - if (likely(!folio_test_locked(folio))) { + if (likely(!folio_test_locked(folio)) && + !(vmf->vma->vm_flags & VM_SEQ_READ)) { mmap_miss = READ_ONCE(ra->mmap_miss); if (mmap_miss) WRITE_ONCE(ra->mmap_miss, --mmap_miss); @@ -3936,10 +3941,15 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, * In such situation, read-ahead is only a waste of IO. * Don't decrease mmap_miss in this scenario to make sure * we can stop read-ahead. + * + * VM_SEQ_READ mappings skip the mmap_miss increment in + * do_sync_mmap_readahead(), so skip the decrement here as + * well to keep the counter symmetric. */ if ((map_ret & VM_FAULT_NOPAGE) && !(vmf->flags & FAULT_FLAG_TRIED) && - !folio_test_workingset(folio)) { + !folio_test_workingset(folio) && + !(vma->vm_flags & VM_SEQ_READ)) { unsigned short mmap_miss; mmap_miss = READ_ONCE(file->f_ra.mmap_miss);