linux-kernel - Re: [PATCH v4 0/4] Optimize mprotect() for large folios

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Message-ID: <7eaee53b-d538-4991-addd-379a380b8ee2@arm.com>
Date: Mon, 30 Jun 2025 11:45:50 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Dev Jain <dev.jain@....com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: david@...hat.com, willy@...radead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
 Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, vbabka@...e.cz,
 jannh@...gle.com, anshuman.khandual@....com, peterx@...hat.com,
 joey.gouly@....com, ioworker0@...il.com, baohua@...nel.org,
 kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
 christophe.leroy@...roup.eu, yangyicong@...ilicon.com,
 linux-arm-kernel@...ts.infradead.org, hughd@...gle.com,
 yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v4 0/4] Optimize mprotect() for large folios

On 30/06/2025 04:33, Dev Jain wrote:
> 
> On 30/06/25 4:35 am, Andrew Morton wrote:
>> On Sat, 28 Jun 2025 17:04:31 +0530 Dev Jain <dev.jain@....com> wrote:
>>
>>> This patchset optimizes the mprotect() system call for large folios
>>> by PTE-batching. No issues were observed with mm-selftests, build
>>> tested on x86_64.
>> um what.  Seems to claim that "selftests still compiles after I messed
>> with stuff", which isn't very impressive ;)  Please clarify?
> 
> Sorry I mean to say that the mm-selftests pass.

I think you're saying you both compiled and ran the mm selftests for arm64. And
additionally you compiled for x86_64? (Just trying to help clarify).


> 
>>
>>> We use the following test cases to measure performance, mprotect()'ing
>>> the mapped memory to read-only then read-write 40 times:
>>>
>>> Test case 1: Mapping 1G of memory, touching it to get PMD-THPs, then
>>> pte-mapping those THPs
>>> Test case 2: Mapping 1G of memory with 64K mTHPs
>>> Test case 3: Mapping 1G of memory with 4K pages
>>>
>>> Average execution time on arm64, Apple M3:
>>> Before the patchset:
>>> T1: 7.9 seconds   T2: 7.9 seconds   T3: 4.2 seconds
>>>
>>> After the patchset:
>>> T1: 2.1 seconds   T2: 2.2 seconds   T3: 4.3 seconds
>> Well that's tasty.
>>
>>> Observing T1/T2 and T3 before the patchset, we also remove the regression
>>> introduced by ptep_get() on a contpte block. And, for large folios we get
>>> an almost 74% performance improvement, albeit the trade-off being a slight
>>> degradation in the small folio case.
>>>