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

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Message-Id: <20250629160549.da922e78d202c510a1ec68f8@linux-foundation.org>
Date: Sun, 29 Jun 2025 16:05:49 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Dev Jain <dev.jain@....com>
Cc: ryan.roberts@....com, 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 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?

> 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.
>