linux-kernel - Re: [PATCH 0/3] Optimize mremap() by PTE-batching

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Message-ID: <81fbf59b-7930-4ee5-94a8-3756a372c4e6@arm.com>
Date: Tue, 6 May 2025 17:23:51 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Anshuman Khandual <anshuman.khandual@....com>, akpm@...ux-foundation.org,
 Liam.Howlett@...cle.com, vbabka@...e.cz, jannh@...gle.com, pfalcato@...e.de,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, david@...hat.com,
 peterx@...hat.com, ryan.roberts@....com, mingo@...nel.org,
 libang.li@...group.com, maobibo@...ngson.cn, zhengqi.arch@...edance.com,
 baohua@...nel.org, willy@...radead.org, ioworker0@...il.com,
 yang@...amperecomputing.com
Subject: Re: [PATCH 0/3] Optimize mremap() by PTE-batching



On 06/05/25 4:14 pm, Lorenzo Stoakes wrote:
> On Tue, May 06, 2025 at 03:52:39PM +0530, Dev Jain wrote:
>>
>>
>> On 06/05/25 2:46 pm, Anshuman Khandual wrote:
>>> On 5/6/25 10:30, Dev Jain wrote:
>>>> Use PTE batching to optimize mremap().
>>>>
>>>> Mapping 512K of memory, memsetting it, remapping it to src + 512K, and
>>>> munmapping it 10,000 times, the average execution time reduces from 1.9 to
>>>> 1.2 seconds, giving a 37% performance optimization. (Apple M3)
>>>
>>> That's impressive improvement. But could you please re-organize the test
>>> description into a pseudo code format or better provide the test program
>>> itself (which should be compact anyways) just to be more clear about the
>>> scenario where this helps.
>>
>> Sure.
> 
> I echo Ashuman's comment, definitely would like to see that.
> 
> And wrt to perf improvement, whether it's a microbenchmark or not, that's a
> great result so well done :) I echo this also!
> 
> However, it'd be good to see some more detail here also, you're kind of missing
> out - everything - about why this improvement happens - what the intent of the
> series is, anything about large folios, under what circumstances you'll see an
> improvement, etc. etc.
> 
> While this might duplicate comments you've made elsewhere, it's mandatory for a
> series, and Andrew is unlikely to take this without it.
> 
> In mm we place the contents of the cover letter in the first commit in the
> series, so it gets stored for posterity also!
> 
> Cheers, Lorenzo

Thanks for your feedback. I'll make the required changes.

> 
>>
>>>
>>>>
>>>> Dev Jain (3):
>>>>     mm: Call pointers to ptes as ptep
>>>>     mm: Add generic helper to hint a large folio
>>>>     mm: Optimize mremap() by PTE batching
>>>>
>>>>    include/linux/pgtable.h | 16 +++++++++++++++
>>>>    mm/mremap.c             | 44 +++++++++++++++++++++++++++--------------
>>>>    2 files changed, 45 insertions(+), 15 deletions(-)
>>>>
>>