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

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Message-ID: <e6b12ef9-1808-45a3-9d64-5c7fd1904b5b@lucifer.local>
Date: Fri, 29 Aug 2025 10:04:01 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Lokesh Gidra <lokeshgidra@...gle.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 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... :)

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo