[PATCH v6 1/2] mm: bypass mmap_miss heuristic for VM_EXEC readahead

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

The mmap_miss heuristic is intended to stop speculative mmap readahead
when a file looks like a random-access workload. That does not fit the
VM_EXEC path very well.

VM_EXEC readahead is already constrained differently from ordinary mmap
read-around: it is bounded by the VMA, uses exec_folio_order() to choose
an order useful for executable mappings, and sets async_size to 0 so it
does not create follow-on readahead. When VM_HUGEPAGE is also present,
the larger readahead is an explicit userspace opt-in.

The mmap_miss counter is decremented from cache-hit paths in
do_async_mmap_readahead() and filemap_map_pages(). Those paths are not
always enough to balance the synchronous miss increments for executable
mappings. In particular, when fault-around is effectively disabled, such
as configurations where fault_around_pages is 1, filemap_map_pages() is
not reached from the fault path. The counter can then become a stale
throttle for VM_EXEC mappings and suppress the readahead behavior that
the executable-specific path is trying to provide.

Skip both mmap_miss increments and decrements for VM_EXEC mappings,
matching the existing VM_SEQ_READ treatment and keeping the counter
accounting symmetric.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 mm/filemap.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index cca20e350c95..a16b33e0fc71 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3339,7 +3339,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 		}
 	}
 
-	if (!(vm_flags & VM_SEQ_READ)) {
+	if (!(vm_flags & (VM_SEQ_READ | VM_EXEC))) {
 		/* Avoid banging the cache line if not needed */
 		mmap_miss = READ_ONCE(ra->mmap_miss);
 		if (mmap_miss < MMAP_LOTSAMISS * 10)
@@ -3434,12 +3434,12 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 	 * 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
+	 * VM_SEQ_READ and VM_EXEC 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)) &&
-	    !(vmf->vma->vm_flags & VM_SEQ_READ)) {
+	    !(vmf->vma->vm_flags & (VM_SEQ_READ | VM_EXEC))) {
 		mmap_miss = READ_ONCE(ra->mmap_miss);
 		if (mmap_miss)
 			WRITE_ONCE(ra->mmap_miss, --mmap_miss);
@@ -3941,14 +3941,14 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		 * 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.
+		 * VM_SEQ_READ and VM_EXEC 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) &&
-		    !(vma->vm_flags & VM_SEQ_READ)) {
+		    !(vma->vm_flags & (VM_SEQ_READ | VM_EXEC))) {
 			unsigned short mmap_miss;
 
 			mmap_miss = READ_ONCE(file->f_ra.mmap_miss);
-- 
2.52.0