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

Jan Kara
2025-05-05T09:56:10Z

On Wed 30-04-25 15:59:17, 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>

...

> @@ -469,6 +469,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
> int err = 0;
> gfp_t gfp = readahead_gfp_mask(mapping);
> unsigned int min_ra_size = max(4, mapping_min_folio_nrpages(mapping));
> + unsigned int new_order = ra->order;
>
> /*
> * Fallback when size < min_nrpages as each folio should be
> @@ -483,6 +484,8 @@ void page_cache_ra_order(struct readahead_control *ractl,
> new_order = min_t(unsigned int, new_order, ilog2(ra->size));
> new_order = max(new_order, min_order);
>
> + ra->order = new_order;
> +
> /* See comment in page_cache_ra_unbounded() */
> nofs = memalloc_nofs_save();
> filemap_invalidate_lock_shared(mapping);
> @@ -525,6 +528,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
> * ->readahead() may have updated readahead window size so we have to
> * check there's still something to read.
> */
> + ra->order = 0;

Hum, so you reset desired folio order if readahead hit some pre-existing
pages in the page cache. Is this really desirable? Why not leave the
desired order as it was for the next request?

> if (ra->size > index - start)
> do_page_cache_ra(ractl, ra->size - (index - start),
> ra->async_size);

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR