linux-kernel - Re: [PATCH v3 2/2] mm: Optimize mremap() by PTE batching

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Message-ID: <170036fa-727c-4d2b-8ed4-0a1b6e4cf8d3@arm.com>
Date: Wed, 28 May 2025 09:02:26 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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,
 anshuman.khandual@....com, willy@...radead.org, ioworker0@...il.com,
 yang@...amperecomputing.com, baolin.wang@...ux.alibaba.com, ziy@...dia.com,
 hughd@...gle.com
Subject: Re: [PATCH v3 2/2] mm: Optimize mremap() by PTE batching


On 27/05/25 10:16 pm, Lorenzo Stoakes wrote:
> On Tue, May 27, 2025 at 10:08:59PM +0530, Dev Jain wrote:
>> On 27/05/25 9:59 pm, Lorenzo Stoakes wrote:
> [snip]
>>> If I invoke split_huge_pmd(), I end up with a bunch of PTEs mapping the same
>>> large folio. The folio itself is not split, so nr_ptes surely will be equal to
>>> something >1 here right?
>>
>> Thanks for elaborating.
>>
>> So,
>>
>> Case 1: folio splitting => nr_ptes = 1 => the question of a/d bit smearing
>> disappears.
>>
>> Case 2: page table splitting => consec PTEs point to the same large folio
>> => nr_ptes > 1 => get_and_clear_full_ptes() will smear a/d bits on the
>> new ptes, which is correct because we are still pointing to the same large
>> folio.
>>
> OK awesome, I thought as much, just wanted to make sure :) we are good then.
>
> The accessed/dirty bits really matter at a folio granularity (and especially
> with respect to reclaim/writeback which both operate at folio level) so the
> smearing as you say is fine.
>
> This patch therefore looks fine, only the trivial comment fixup.
>
> I ran the series on my x86-64 setup (fwiw) with no build/mm selftest errors.

Thanks!


>
> Sorry to be a pain but could you respin with the commit message for this patch
> updated to explicitly mention that the logic applies for the non-contPTE split
> PTE case (and therefore also helps performance there)? That and the trivial
> thing of dropping that comment.

What do you mean by the non-contpte case? In that case the PTEs do not point
to the same folio or are misaligned, and there will be no optimization. This
patch is optimizing two things: 1) ptep_get() READ_ONCE accesses 2) reduction
in number of TLBIs for contig blocks, both of which happen in the contpte case.

In general, the patch should have a minor improvement on other arches because
we are detecting a batch and processing it together, thus saving on a few
function calls, but the main benefit is for arm64.

>
> Then we should be good for a tag unless somebody else spots something
> egregious :)
>
> Thanks for this! Good improvement.
>
> [snip]
>
> Cheers, Lorenzo