linux-kernel - Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch

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Message-ID: <CAGsJ_4w_c3AY1Nw7EhYH3rf0jjqgZ6AYLdr3xuonF9SNgWckDg@mail.gmail.com>
Date: Mon, 11 Aug 2025 14:52:51 +0800
From: Barry Song <21cnbao@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>, 
	Dev Jain <dev.jain@....com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on
 mremap folio pte batch

On Mon, Aug 11, 2025 at 12:57 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Mon, Aug 11, 2025 at 10:40:50AM +0800, Barry Song wrote:
> > On Fri, Aug 8, 2025 at 2:59 AM Lorenzo Stoakes
> > > The expectation by those discussing this from the start was that
> > > vm_normal_folio() (invoked by mremap_folio_pte_batch()) would likely be the
> > > culprit due to having to retrieve memory from the vmemmap (which mremap()
> > > page table moves does not otherwise do, meaning this is inevitably cold
> > > memory).
> >
> > If vm_normal_folio() is so expensive, does that mean it negates the
> > benefits that commit f822a9a81a31 (“mm: optimize mremap() by PTE
> > batching”) was originally intended to achieve through PTE batching?
>
> Not for arm64 apparently. And the hint check introduces here should avoid
> regressions even there when small folios are in place.

I still don’t understand why this is fine on arm64. We do have faster
folio_pte_batch(), get_and_clear_ptes(), and set_ptes() with contpte, but
are those benefits really enough to outweigh the disadvantage of
vm_normal_folio(), given those PTEs are likely in the same cacheline?

Unless the previous contpte_try_unfold() was very costly and removing it yielded
a significant improvement, it’s difficult to see how the benefits would outweigh
the drawbacks of vm_normal_folio(). Does this imply that there was already a
regression in mremap() caused by contpte_try_unfold() before?
And that Dev’s patch is essentially a fix for this regression on arm64?

Sorry, maybe I’m talking too much, but I’m curious about the whole story:-)

>
> In similar series to these in other areas, it appears we need the folio
> anyway so there is no additional overhead to deal with, in mremap() you'd
> otherwise just be looking at page tables which makes so this egregious
> here.
>

Thanks
Barry