Re: [RFC PATCH 3/4] mm/shmem: optimize file read with folio batching

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 5/15/26 5:47 PM, Chi Zhiling wrote:
> From: Chi Zhiling <chizhiling@kylinos.cn>
> 
> Optimize shmem file read by implementing folio batching in the read
> iteration path. Only uptodate folios are added to the batch, ensuring
> all folios in the batch are valid and ready for use without additional
> checking.
> 
> Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
> ---
>   mm/shmem.c | 102 ++++++++++++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 93 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 767610f78d0d..4bc4e463ca97 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3348,16 +3348,100 @@ shmem_write_end(const struct kiocb *iocb, struct address_space *mapping,
>   	return copied;
>   }
>   
> +static pgoff_t shmem_get_read_batch(struct address_space *mapping,
> +		pgoff_t index, pgoff_t max, struct folio_batch *fbatch)
> +{
> +	XA_STATE(xas, &mapping->i_pages, index);
> +	struct folio *folio;
> +	pgoff_t end = max;
> +
> +	rcu_read_lock();
> +	xas_for_each(&xas, folio, max) {
> +		if (xas_retry(&xas, folio))
> +			continue;
> +		if (xa_is_value(folio)) {
> +			end = xas.xa_index;
> +			break;
> +		}
> +		if (!folio_try_get(folio))
> +			goto retry;
> +
> +		if (unlikely(folio != xas_reload(&xas)))
> +			goto put_folio;
> +
> +		end = folio_next_index(folio);
> +
> +		if (!folio_test_uptodate(folio)) {
> +			xas_advance(&xas, end - 1);
> +			folio_put(folio);
> +			continue;
> +		}
> +		if (!folio_batch_add(fbatch, folio))
> +			break;
> +		xas_advance(&xas, end - 1);
> +		continue;
> +put_folio:
> +		folio_put(folio);
> +retry:
> +		xas_reset(&xas);
> +	}
> +	rcu_read_unlock();
> +
> +	return end;
> +}

I haven't looked at this patch in detail yet, but I feel this part 
should belong in mm/filemap.c, or perhaps mm/filemap.c already has a 
similar function implemented? Let's wait for Matthew's comments on this.