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

From: Pedro Falcato <[email-masked]>
Date: Fri, 29 May 2026 10:47:05 +0100

On Thu, May 28, 2026 at 09:55:19AM -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 <[email-masked]>
> Reviewed-by: Jan Kara <[email-masked]>
> Reviewed-by: Kiryl Shutsemau (Meta) <[email-masked]>

Reviewed-by: Pedro Falcato <[email-masked]>

This is reasonable, thanks.

> ---
>  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
> 
> 

-- 
Pedro