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

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Message-ID: <c072049b-d6ef-47ce-b281-006ae0f721b3@redhat.com>
Date: Fri, 25 Oct 2024 21:08:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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

>>
>> 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.

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

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! :)

-- 
Cheers,

David / dhildenb