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

Jan Kara
2025-05-06T13:12:06Z

On Tue 06-05-25 10:53:07, Ryan Roberts wrote:
> On 05/05/2025 10:52, Jan Kara wrote:
> > 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?
>
> My aim was to not let order grow unbounded. When the filesystem doesn't support
> large folios we end up here (from the "goto fallback") and without this, order
> will just grow and grow (perhaps it doesn't matter though). I think we should
> keep this.

Yes, I agree that should be kept.

>
> But I guess your point is that we can also end up here when the filesystem does
> support large folios but there is an error. In thta case, yes, I'll change to
> not reset order to 0; it has already been fixed up earlier in this path.

Right.

> How's this:
>
> ---8<---
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 18972bc34861..0054ca18a815 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -475,8 +475,10 @@ void page_cache_ra_order(struct readahead_control *ractl,
> * 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)
> + if (!mapping_large_folio_support(mapping) || ra->size < min_ra_size) {
> + ra->order = 0;
> goto fallback;
> + }
>
> limit = min(limit, index + ra->size - 1);
>
> @@ -528,7 +530,6 @@ 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;
> if (ra->size > index - start)
> do_page_cache_ra(ractl, ra->size - (index - start),
> ra->async_size);

Yes, this looks good to me.

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