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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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