[RFC PATCH v4 4/5] mm/readahead: Store folio order in struct file_ra_state
Ryan Roberts
ryan.roberts at arm.com
Tue May 6 08:06:21 PDT 2025
On 06/05/2025 15:24, David Hildenbrand wrote:
> On 06.05.25 12:03, Ryan Roberts wrote:
>> 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 at 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.
>
> I'm fine with it staying that way; I was merely trying to make sense of it all ...
>
>
> ... maybe a better description of the parameters might make the semantics easier
> to grasp.
>
> ""most recent" includes "current" once passed into page_cache_ra_order()"
>
> is *really* hard to digest :)
>
>>
>>>
>>> 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...
>
> Agreed. As above, I think we might do better with the description of these
> parameters in general ...
>
> or even document how page_cache_ra_order() acts on these inputs?
OK let me try to work something up for the next version...
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