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

[Jan Kara <jack@suse.cz>]

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