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

Ryan Roberts
2025-05-06T11:47:17Z

On 05/05/2025 11:08, David Hildenbrand wrote:
> 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"?

As you know, I'm crap at naming, but...

start, size, async_size and order make up the parameters for the "most recent"
readahead request. Where "most recent" includes "current" once passed into
page_cache_ra_order(). The others don't include "ra" or "prev" in their name so
wasn't sure it was necessary here.

ra_pages is a bit different; that's not part of the request, it's a (dynamic)
ceiling to use when creating requests.

Personally I'd leave it as is, but no strong opinion.

>
> 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?

You mean continue to pass new_order as a parameter to page_cache_ra_order()? The
reason I did it the way I'm doing it is because I thought it would be weird for
the caller of page_cache_ra_order() to set up all the parameters (start, size,
async_size) of the request except for order...

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

But if you think your suggestion makes things clearer, then that's fine by me.

Thanks,
Ryan

>
> Again, just a thought ...
>