[RFC PATCH v4 1/5] mm/readahead: Honour new_order in page_cache_ra_order()

[Pankaj Raghav (Samsung)]

Hey Ryan,

On Wed, Apr 30, 2025 at 03:59:14PM +0100, Ryan Roberts wrote:
> FOLIO  0x0001a000  0x0001b000       4096       26       27      1      0
> FOLIO  0x0001b000  0x0001c000       4096       27       28      1      0
> FOLIO  0x0001c000  0x0001d000       4096       28       29      1      0
> FOLIO  0x0001d000  0x0001e000       4096       29       30      1      0
> FOLIO  0x0001e000  0x0001f000       4096       30       31      1      0
> FOLIO  0x0001f000  0x00020000       4096       31       32      1      0
> FOLIO  0x00020000  0x00024000      16384       32       36      4      2
> FOLIO  0x00024000  0x00028000      16384       36       40      4      2
> FOLIO  0x00028000  0x0002c000      16384       40       44      4      2
> FOLIO  0x0002c000  0x00030000      16384       44       48      4      2
> ...
> 
> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
> ---
>  mm/readahead.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 6a4e96b69702..8bb316f5a842 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -479,9 +479,6 @@ void page_cache_ra_order(struct readahead_control *ractl,
>  

So we always had a fallback to do_page_cache_ra() if the size of the
readahead is less than 4 pages (16k). I think this was there because we
were adding `2` to the new_order:

unsigned int min_ra_size = max(4, mapping_min_folio_nrpages(mapping));

/*
 * Fallback when size < min_nrpages as each folio should be
 * at least min_nrpages anyway.
 */
if (!mapping_large_folio_support(mapping) || ra->size < min_ra_size)
	goto fallback;

>  	limit = min(limit, index + ra->size - 1);
>  
> -	if (new_order < mapping_max_folio_order(mapping))
> -		new_order += 2;

Now that you have moved this, we could make the lhs of the max to be 2
(8k) instead of 4(16k).

- unsigned int min_ra_size = max(4, mapping_min_folio_nrpages(mapping));
+ unsigned int min_ra_size = max(2, mapping_min_folio_nrpages(mapping));

I think if we do that, we might ramp up to 8k sooner rather than jumping
from 4k to 16k directly?

> -
>  	new_order = min(mapping_max_folio_order(mapping), new_order);
>  	new_order = min_t(unsigned int, new_order, ilog2(ra->size));
>  	new_order = max(new_order, min_order);
> @@ -683,6 +680,7 @@ void page_cache_async_ra(struct readahead_control *ractl,
>  	ra->size = get_next_ra_size(ra, max_pages);
>  	ra->async_size = ra->size;
>  readit:
> +	order += 2;
>  	ractl->_index = ra->start;
>  	page_cache_ra_order(ractl, ra, order);
>  }
> -- 
> 2.43.0
> 

--
Pankaj