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

[David Hildenbrand]

On 15.08.25 20:32, 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@xxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
>   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);
> +	}

Makes sense to me, bud I am no readahead expert.

--
Cheers

David / dhildenb