linux-kernel - Re: [PATCH v3 1/5] mm: pagewalk: add the ability to install PTEs

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Message-ID: <dc180613-1b55-4e8d-b365-a501cc076474@lucifer.local>
Date: Sat, 26 Oct 2024 08:42:30 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
        "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Muchun Song <muchun.song@...ux.dev>,
        Richard Henderson <richard.henderson@...aro.org>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...nel.org>,
        linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
        linux-kselftest@...r.kernel.org,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        Jeff Xu <jeffxu@...omium.org>, Christoph Hellwig <hch@...radead.org>,
        linux-api@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v3 1/5] mm: pagewalk: add the ability to install PTEs

On Fri, Oct 25, 2024 at 09:08:24PM +0200, David Hildenbrand wrote:
> > >
> > > We already discussed in the past that we need a better and more efficient
> > > way to walk page tables. I have part of that on my TODO list, but I'm
> > > getting distracted.
> >
> > Yes I remember an LSF session on this, it's a really obvious area of improvement
> > that stands out at the moment for sure.
> >
> > Having worked several 12+ hour days in a row though recently I can relate to
> > workload making this difficult though :)
>
> Yes :)
>
> >
> > >
> > > *Inserting* (not walking/modifying existing things as most users to) as done
> > > in this patch is slightly different though, likely "on thing that fits all"
> > > will not apply to all page table walker user cases.
> >
> > Yeah, there's also replace scenarios which then have to do egregious amounts of
> > work to make sure we do everything right, in fact there's duplicates of this in
> > mm/madvise.c *grumble grumble*.
> >
> > >
> > > --
> > > Cheers,
> > >
> > > David / dhildenb
> > >
> >
> > OK so I guess I'll hold off my TODOs on this as you are looking in this area and
> > I trust you :)
>
> It will probably take me a while until I get to it, though. I'd focus on
> walking (and batching what we can) first, then on top modifying existing
> entries.

Yeah entirely understandable and that's the right order I think, the
modifying path is the trickier one especially with all the special cases
all over the place...

>
> The "install something where there is nothing yet" (incl. populating fresh
> page tables etc.) case probably deserves a separate "walker".

Yes this would avoid all the heavy handling a replace handler needs.

>
> If you end up having spare cycles and want to sync on a possible design for
> some part of that bigger task -- removing the old pagewalk logic -- please
> do reach out! :)

Thanks, I may have a play when I have a brief moment, as I feel quite
strongly we need to attack this (as do you I feel! :) will send some RFC or
some thoughts or whatever should I do so!

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

Cheers!