From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8983E198A08; Tue, 6 May 2025 15:06:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746543989; cv=none; b=qhs6noCOVO1vVpElxAkbAJIt0Z6BKJxHAMHj0PeVsWos7pyZ4GptXePeq9BmHH6HmtvU7i1KOLlagtrxZyekWaFpZ2FIsYPukYP2dzIjUFLIhoL2/7o7roId/6l9ScIWNvWKBNQxdiKqhanhYzXIlf51pV/CDwBaB8cXvrgr8/U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746543989; c=relaxed/simple; bh=7bNdS1i9+vkYuxnUFivUGESM1y2DIHExk6zpg9CpGWk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Nb+6CqOClN1A+b/jQSirEmOzMksjZHM9hNWp0RYC6kPdnAfTEqVtJuNnmCcfpWWJsYJGyLo7P6v0vcqY8PKhzHZcuSF9rR+Ah2Oge+2vJB2AmhAM0hUvwvgms98hx0dyM05RuMzK0D9R6ZbG7ANOcahRzvxKDiYHh1a8JMB4t7g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5D9FB339; Tue, 6 May 2025 08:06:16 -0700 (PDT) Received: from [10.1.29.178] (XHFQ2J9959.cambridge.arm.com [10.1.29.178]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3C0A93F58B; Tue, 6 May 2025 08:06:23 -0700 (PDT) Message-ID: Date: Tue, 6 May 2025 16:06:21 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v4 4/5] mm/readahead: Store folio order in struct file_ra_state Content-Language: en-GB To: David Hildenbrand , Andrew Morton , "Matthew Wilcox (Oracle)" , Alexander Viro , Christian Brauner , Jan Kara , Dave Chinner , Catalin Marinas , Will Deacon , Kalesh Singh , Zi Yan Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org References: <20250430145920.3748738-1-ryan.roberts@arm.com> <20250430145920.3748738-5-ryan.roberts@arm.com> <2b1ea3d9-6c9b-4700-ae21-5f65565a995a@arm.com> <5ea287f0-24cb-4ad4-8448-6e397fbf1ec8@redhat.com> From: Ryan Roberts In-Reply-To: <5ea287f0-24cb-4ad4-8448-6e397fbf1ec8@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 >>>> --- >>>>    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...