Re: [PATCH] mm: readahead: improve mmap_miss heuristic for concurrent faults

[Mateusz Guzik <mjguzik@gmail.com>]

On Fri, Aug 15, 2025 at 11:32:24AM -0700, Roman Gushchin wrote:
> If two or more threads of an application faulting on the same folio,
> the mmap_miss counter can be decreased multiple times. It breaks the
> mmap_miss heuristic and keeps the readahead enabled even under extreme
> levels of memory pressure.
> 
> It happens often if file folios backing a multi-threaded application
> are getting evicted and re-faulted.
> 
> Fix it by skipping decreasing mmap_miss if the folio is locked.
> 
> This change was evaluated on several hundred thousands hosts in Google's
> production over a couple of weeks. The number of containers being
> stuck in a vicious reclaim cycle for a long time was reduced several
> fold (~10-20x), as well as the overall fleet-wide cpu time spent in
> direct memory reclaim was meaningfully reduced. No regressions were
> observed.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: linux-mm@kvack.org
> ---
>  mm/filemap.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c21e98657e0b..983ba1019674 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3324,9 +3324,17 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>  	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
>  		return fpin;
>  
> -	mmap_miss = READ_ONCE(ra->mmap_miss);
> -	if (mmap_miss)
> -		WRITE_ONCE(ra->mmap_miss, --mmap_miss);
> +	/*
> +	 * If the folio is locked, we're likely racing against another fault.
> +	 * 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().
> +	 */
> +	if (likely(!folio_test_locked(folio))) {
> +		mmap_miss = READ_ONCE(ra->mmap_miss);
> +		if (mmap_miss)
> +			WRITE_ONCE(ra->mmap_miss, --mmap_miss);
> +	}

I'm not an mm person.

The comment implies the change fixes the race, but it is not at all
clear to me how.

Does it merely make it significantly less likely?