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 <usama.arif@linux.dev>
Closes: https://lore.kernel.org/all/8edc8cd0-f65c-4456-9b3f-362e744c9a96@linux.dev/
Reviewed-by: William Kucharski <william.kucharski@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 12 insertions, 2 deletions

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);