Re: [RFC PATCH v4 4/5] mm/readahead: Store folio order in struct file_ra_state

David Hildenbrand
2025-05-05T10:11:14Z

On 30.04.25 16:59, Ryan Roberts wrote:
> Previously the folio order of the previous readahead request was
> inferred from the folio who's readahead marker was hit. But due to the
> way we have to round to non-natural boundaries sometimes, this first
> folio in the readahead block is often smaller than the preferred order
> for that request. This means that for cases where the initial sync
> readahead is poorly aligned, the folio order will ramp up much more
> slowly.
>
> So instead, let's store the order in struct file_ra_state so we are not
> affected by any required alignment. We previously made enough room in
> the struct for a 16 order field. This should be plenty big enough since
> we are limited to MAX_PAGECACHE_ORDER anyway, which is certainly never
> larger than ~20.
>
> Since we now pass order in struct file_ra_state, page_cache_ra_order()
> no longer needs it's new_order parameter, so let's remove that.
>
> Worked example:
>
> Here we are touching pages 17-256 sequentially just as we did in the
> previous commit, but now that we are remembering the preferred order
> explicitly, we no longer have the slow ramp up problem. Note
> specifically that we no longer have 2 rounds (2x ~128K) of order-2
> folios:
>
> TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER RA
> ----- ---------- ---------- ---------- ------- ------- ----- ----- --
> HOLE 0x00000000 0x00001000 4096 0 1 1
> 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 0x00021000 4096 32 33 1 0
> FOLIO 0x00021000 0x00022000 4096 33 34 1 0
> FOLIO 0x00022000 0x00024000 8192 34 36 2 1
> 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
> FOLIO 0x000e0000 0x00100000 131072 224 256 32 5
> FOLIO 0x00100000 0x00120000 131072 256 288 32 5
> FOLIO 0x00120000 0x00140000 131072 288 320 32 5 Y
> HOLE 0x00140000 0x00800000 7077888 320 2048 1728
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> include/linux/fs.h | 2 ++
> mm/filemap.c | 6 ++++--
> mm/internal.h | 3 +--
> mm/readahead.c | 18 +++++++++++-------
> 4 files changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 44362bef0010..cde482a7270a 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1031,6 +1031,7 @@ struct fown_struct {
> * and so were/are genuinely "ahead". Start next readahead when
> * the first of these pages is accessed.
> * @ra_pages: Maximum size of a readahead request, copied from the bdi.
> + * @order: Preferred folio order used for most recent readahead.

Looking at other members, and how it relates to the other members,
should we call this something like "ra_prev_order" / "prev_ra_order" to
distinguish it from !ra members and indicate the "most recent" semantics
similar to "prev_pos"?

Just a thought while digging through this patch ...

...

> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3222,7 +3222,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> if (!(vm_flags & VM_RAND_READ))
> ra->size *= 2;
> ra->async_size = HPAGE_PMD_NR;
> - page_cache_ra_order(&ractl, ra, HPAGE_PMD_ORDER);
> + ra->order = HPAGE_PMD_ORDER;
> + page_cache_ra_order(&ractl, ra);
> return fpin;
> }
> #endif
> @@ -3258,8 +3259,9 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
> ra->size = ra->ra_pages;
> ra->async_size = ra->ra_pages / 4;
> + ra->order = 0;
> ractl._index = ra->start;
> - page_cache_ra_order(&ractl, ra, 0);
> + page_cache_ra_order(&ractl, ra);
> return fpin;
> }

Why not let page_cache_ra_order() consume the order and update ra->order
(or however it will be called :) ) internally?

That might make at least the "most recent readahead" semantics of the
variable clearer.

Again, just a thought ...

--
Cheers,

David / dhildenb