linux-kernel - Re: [DISCUSSION] Unconditionally lock folios when calling rmap

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Message-ID: <CA+EESO5FnraOUV=CzcD6t_e086pC7rDZuVv4w2ng==7KacA=Pg@mail.gmail.com>
Date: Tue, 2 Sep 2025 11:59:23 -0700
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: David Hildenbrand <david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Harry Yoo <harry.yoo@...cle.com>, Zi Yan <ziy@...dia.com>, Barry Song <21cnbao@...il.com>, 
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>, Peter Xu <peterx@...hat.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Kalesh Singh <kaleshsingh@...gle.com>, 
	android-mm <android-mm@...gle.com>, linux-kernel <linux-kernel@...r.kernel.org>, 
	Jann Horn <jannh@...gle.com>, Rik van Riel <riel@...riel.com>, Vlastimil Babka <vbabka@...e.cz>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [DISCUSSION] Unconditionally lock folios when calling rmap_walk()

On Fri, Aug 29, 2025 at 2:04 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Fri, Aug 29, 2025 at 10:42:45AM +0200, David Hildenbrand wrote:
> > > >
> > > > I do wonder if we can identify this case and handle things differently.
> > > >
> > > > Perhaps even saying 'try and get the rmap lock, but if there's "too much"
> > > > contention, grab the folio lock.
> > >
> > > Can you please elaborate what you mean? Where do you mean we can
> > > possibly do something like this?
> > >
> > > UFFD move only works on PageAnonExclusive folios. So, would it help
> > > (in terms of avoiding contention) if we were to change the condition:
> >
> > I think we shouldn't be using PAE here. Once could consider using
> > folio_maybe_mapped_shared(), and assume contention on the folio lock if it
> > is maybe mapped shared.
>
> Interesting!
>
> >
> > But the real question is with whom we would be contending for the folio
> > lock.
> >
> > Is it really other processes mapping that folio? I'm not so sure.
>
> Yeah, I might go off and do some research myself on this, actually. Nail down
> wehre this might actually happen.
>
> Generally I'm softening on this and maybe we're good with the proposed change.
>
> But still want to be super careful here... :)
>
Anxiously waiting for your assessment. Fingers crossed :)
> >
> > --
> > Cheers
> >
> > David / dhildenb
> >
>
> Cheers, Lorenzo