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

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Message-ID: <67cf2131-da6b-4a1d-828e-52f0ff7fd0fb@lucifer.local>
Date: Tue, 27 May 2025 17:46:41 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....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 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.

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.

Then we should be good for a tag unless somebody else spots something
egregious :)

Thanks for this! Good improvement.

[snip]

Cheers, Lorenzo