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

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Message-ID: <a089ff3b-119e-44a0-a780-3abca01886f7@redhat.com>
Date: Thu, 24 Oct 2024 09:45:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: 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 24.10.24 09:34, Lorenzo Stoakes wrote:
> On Wed, Oct 23, 2024 at 04:04:05PM -0700, Andrew Morton wrote:
>> On Wed, 23 Oct 2024 17:24:38 +0100 Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:
>>
>>>
>>> ...
>>>
>>> Existing mechanism for performing a walk which also installs page table
>>> entries if necessary are heavily duplicated throughout the kernel,
>>
>> How complicated is it to migrate those to use this?
> 
> I would say probably somewhat difficult as very often people are doing quite
> custom things, but I will take a look at seeing if we can't make things a little
> more generic.
> 
> I am also mildly motivated to look at trying to find a generic way to do
> replaces...
> 
> Both on the TODO!

I'm not super happy about extending the rusty old pagewalk API, because 
it's inefficient (indirect calls) and not future proof (batching, large 
folios).

But I see how we ended up with this patch, and it will be easy to 
convert to something better once we have it.

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.

*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.

-- 
Cheers,

David / dhildenb