Re: [PATCH v7 1/2] mm: bypass mmap_miss heuristic for VM_EXEC readahead

On Mon, Jun 01, 2026 at 03:21:17AM -0700, Usama Arif wrote:

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>

Reviewed-by: Oscar Salvador (SUSE) <osalvador@kernel.org>


-- 
Oscar Salvador
SUSE Labs