Re: [PATCH v2 1/5] mm/filemap: reduce unnecessary xarray lookups when read cached pages

[Chi Zhiling]

Hi, Matthew

On 6/1/26 8:51 PM, Matthew Wilcox wrote:
> On Mon, Jun 01, 2026 at 01:57:00PM +0800, Chi Zhiling wrote:
> > From: Chi Zhiling <chizhiling@xxxxxxxxxx>
> >
> > When reading small amounts of data from the page cache, only a single
> > folio is typically returned from filemap_read_get_batch(). In this case,
> > calling xas_advance() or xas_next() after adding the folio to the batch
> > is unnecessary and only introduces extra branches.
>
> I don't think xas_advance() is all that expensive.  The expensive part
> is going to be calling xas_next() and realising that the folio now lies
> beyond the required range.  I think you'll get just as much benefit from
> doing this:
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 4e636647100c..578b03bd5514 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2459,11 +2459,14 @@ static void filemap_get_read_batch(struct address_space *mapping,
>   	XA_STATE(xas, &mapping->i_pages, index);
>   	struct folio *folio;
>   
> +	if (index > max)
> +		return;
> +
>   	rcu_read_lock();
>   	for (folio = xas_load(&xas); folio; folio = xas_next(&xas)) {
>   		if (xas_retry(&xas, folio))
>   			continue;
> -		if (xas.xa_index > max || xa_is_value(folio))
> +		if (xa_is_value(folio))
>   			break;
>   		if (xa_is_sibling(folio))
>   			break;
> @@ -2480,6 +2483,8 @@ static void filemap_get_read_batch(struct address_space *mapping,
>   		if (folio_test_readahead(folio))
>   			break;
>   		xas_advance(&xas, folio_next_index(folio) - 1);
> +		if (xas.index >= max)
> +			break;
>   		continue;
>   put_folio:
>   		folio_put(folio);
> >

Looks good. I'll include it in the next version and make the same change 
in patch 2/5.

Thanks!