[PATCH] mm: make mmap_miss accounting symmetric for VM_SEQ_READ

[Usama Arif <usama.arif@linux.dev>]

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.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/filemap.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 5aaba0d3e81d..cca20e350c95 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3433,8 +3433,13 @@ 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);
@@ -3935,10 +3940,15 @@ 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);
-- 
2.52.0