[PATCH v5 1/2] mm: bypass mmap_miss heuristic for VM_EXEC readahead

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

The mmap_miss counter in do_sync_mmap_readahead() tracks whether
readahead is useful for mmap'd file access. It is incremented by 1 on
every page cache miss in do_sync_mmap_readahead(), and decremented in
two places:

  - filemap_map_pages(): decremented by N for each of N pages
    successfully mapped via fault-around (pages found already in cache,
    evidence readahead was useful). Only pages not in the workingset
    count as hits.

  - do_async_mmap_readahead(): decremented by 1 when a page with
    PG_readahead is found in cache.

When the counter exceeds MMAP_LOTSAMISS (100), all readahead is
disabled, including the targeted VM_EXEC readahead [1] that requests
large folio orders for contpte mapping.

On arm64 with 64K base pages, both decrement paths are inactive:

  1. filemap_map_pages() is never called because fault_around_pages
     (65536 >> PAGE_SHIFT = 1) disables should_fault_around(), which
     requires fault_around_pages > 1. With only 1 page in the
     fault-around window, there is nothing "around" to map.

  2. do_async_mmap_readahead() never fires for exec mappings because
     exec readahead sets async_size = 0, so no PG_readahead markers
     are placed.

With no decrements, mmap_miss monotonically increases past
MMAP_LOTSAMISS after 100 page faults, disabling all subsequent
exec readahead.

Fix this by excluding VM_EXEC VMAs from the mmap_miss logic, similar
to how VM_SEQ_READ is already excluded. The exec readahead path is
targeted (one folio at the fault location, async_size=0), not
speculative prefetch, so the mmap_miss heuristic designed to throttle
wasteful speculative readahead should not apply to it.

[1] https://lore.kernel.org/all/20250430145920.3748738-6-ryan.roberts@arm.com/

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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 5aaba0d3e81d..f45a1b74870d 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)
-- 
2.52.0