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

Jan Kara
2025-05-05T08:51:41Z

On Wed 30-04-25 15:59:14, Ryan Roberts wrote:
> page_cache_ra_order() takes a parameter called new_order, which is
> intended to express the preferred order of the folios that will be
> allocated for the readahead operation. Most callers indeed call this
> with their preferred new order. But page_cache_async_ra() calls it with
> the preferred order of the previous readahead request (actually the
> order of the folio that had the readahead marker, which may be smaller
> when alignment comes into play).
>
> And despite the parameter name, page_cache_ra_order() always treats it
> at the old order, adding 2 to it on entry. As a result, a cold readahead
> always starts with order-2 folios.
>
> Let's fix this behaviour by always passing in the *new* order.
>
> Worked example:
>
> Prior to the change, mmaping an 8MB file and touching each page
> sequentially, resulted in the following, where we start with order-2
> folios for the first 128K then ramp up to order-4 for the next 128K,
> then get clamped to order-5 for the rest of the file because pa_pages is
> limited to 128K:
>
> TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER
> ----- ---------- ---------- --------- ------- ------- ----- -----
> FOLIO 0x00000000 0x00004000 16384 0 4 4 2
> FOLIO 0x00004000 0x00008000 16384 4 8 4 2
> FOLIO 0x00008000 0x0000c000 16384 8 12 4 2
> FOLIO 0x0000c000 0x00010000 16384 12 16 4 2
> FOLIO 0x00010000 0x00014000 16384 16 20 4 2
> FOLIO 0x00014000 0x00018000 16384 20 24 4 2
> FOLIO 0x00018000 0x0001c000 16384 24 28 4 2
> FOLIO 0x0001c000 0x00020000 16384 28 32 4 2
> FOLIO 0x00020000 0x00030000 65536 32 48 16 4
> FOLIO 0x00030000 0x00040000 65536 48 64 16 4
> FOLIO 0x00040000 0x00060000 131072 64 96 32 5
> FOLIO 0x00060000 0x00080000 131072 96 128 32 5
> FOLIO 0x00080000 0x000a0000 131072 128 160 32 5
> FOLIO 0x000a0000 0x000c0000 131072 160 192 32 5
> ...
>
> After the change, the same operation results in the first 128K being
> order-0, then we start ramping up to order-2, -4, and finally get
> clamped at order-5:
>
> TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER
> ----- ---------- ---------- --------- ------- ------- ----- -----
> FOLIO 0x00000000 0x00001000 4096 0 1 1 0
> FOLIO 0x00001000 0x00002000 4096 1 2 1 0
> FOLIO 0x00002000 0x00003000 4096 2 3 1 0
> FOLIO 0x00003000 0x00004000 4096 3 4 1 0
> FOLIO 0x00004000 0x00005000 4096 4 5 1 0
> FOLIO 0x00005000 0x00006000 4096 5 6 1 0
> FOLIO 0x00006000 0x00007000 4096 6 7 1 0
> FOLIO 0x00007000 0x00008000 4096 7 8 1 0
> FOLIO 0x00008000 0x00009000 4096 8 9 1 0
> FOLIO 0x00009000 0x0000a000 4096 9 10 1 0
> FOLIO 0x0000a000 0x0000b000 4096 10 11 1 0
> FOLIO 0x0000b000 0x0000c000 4096 11 12 1 0
> FOLIO 0x0000c000 0x0000d000 4096 12 13 1 0
> FOLIO 0x0000d000 0x0000e000 4096 13 14 1 0
> FOLIO 0x0000e000 0x0000f000 4096 14 15 1 0
> FOLIO 0x0000f000 0x00010000 4096 15 16 1 0
> FOLIO 0x00010000 0x00011000 4096 16 17 1 0
> FOLIO 0x00011000 0x00012000 4096 17 18 1 0
> FOLIO 0x00012000 0x00013000 4096 18 19 1 0
> FOLIO 0x00013000 0x00014000 4096 19 20 1 0
> FOLIO 0x00014000 0x00015000 4096 20 21 1 0
> FOLIO 0x00015000 0x00016000 4096 21 22 1 0
> FOLIO 0x00016000 0x00017000 4096 22 23 1 0
> FOLIO 0x00017000 0x00018000 4096 23 24 1 0
> FOLIO 0x00018000 0x00019000 4096 24 25 1 0
> FOLIO 0x00019000 0x0001a000 4096 25 26 1 0
> 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
> FOLIO 0x00030000 0x00034000 16384 48 52 4 2
> FOLIO 0x00034000 0x00038000 16384 52 56 4 2
> FOLIO 0x00038000 0x0003c000 16384 56 60 4 2
> FOLIO 0x0003c000 0x00040000 16384 60 64 4 2
> FOLIO 0x00040000 0x00050000 65536 64 80 16 4
> FOLIO 0x00050000 0x00060000 65536 80 96 16 4
> FOLIO 0x00060000 0x00080000 131072 96 128 32 5
> FOLIO 0x00080000 0x000a0000 131072 128 160 32 5
> FOLIO 0x000a0000 0x000c0000 131072 160 192 32 5
> FOLIO 0x000c0000 0x000e0000 131072 192 224 32 5
> ...
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

Makes sense and looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> 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,
>
> limit = min(limit, index + ra->size - 1);
>
> - if (new_order < mapping_max_folio_order(mapping))
> - new_order += 2;
> -
> 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
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR