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Message-ID: <5ea287f0-24cb-4ad4-8448-6e397fbf1ec8@redhat.com>
Date: Tue, 6 May 2025 16:24:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
Dave Chinner <david@...morbit.com>, Catalin Marinas
<catalin.marinas@....com>, Will Deacon <will@...nel.org>,
Kalesh Singh <kaleshsingh@...gle.com>, Zi Yan <ziy@...dia.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v4 4/5] mm/readahead: Store folio order in struct
file_ra_state
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@....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?
--
Cheers,
David / dhildenb
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