linux-kernel - Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes

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Message-ID: <9927f9a3-efba-4053-8384-cc69c7949ea6@intel.com>
Date: Wed, 2 Oct 2024 10:35:24 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Anthony Yznaga <anthony.yznaga@...cle.com>, akpm@...ux-foundation.org,
 willy@...radead.org, markhemm@...glemail.com, viro@...iv.linux.org.uk,
 david@...hat.com, khalid@...nel.org
Cc: andreyknvl@...il.com, luto@...nel.org, brauner@...nel.org, arnd@...db.de,
 ebiederm@...ssion.com, catalin.marinas@....com, linux-arch@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhiramat@...nel.org,
 rostedt@...dmis.org, vasily.averin@...ux.dev, xhao@...ux.alibaba.com,
 pcc@...gle.com, neilb@...e.de, maz@...nel.org,
 David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes

We were just chatting about this on David Rientjes's MM alignment call.
I thought I'd try to give a little brain

Let's start by thinking about KVM and secondary MMUs.  KVM has a primary
mm: the QEMU (or whatever) process mm.  The virtualization (EPT/NPT)
tables get entries that effectively mirror the primary mm page tables
and constitute a secondary MMU.  If the primary page tables change,
mmu_notifiers ensure that the changes get reflected into the
virtualization tables and also that the virtualization paging structure
caches are flushed.

msharefs is doing something very similar.  But, in the msharefs case,
the secondary MMUs are actually normal CPU MMUs.  The page tables are
normal old page tables and the caches are the normal old TLB.  That's
what makes it so confusing: we have lots of infrastructure for dealing
with that "stuff" (CPU page tables and TLB), but msharefs has
short-circuited the infrastructure and it doesn't work any more.

Basically, I think it makes a lot of sense to check what KVM (or another
mmu_notifier user) is doing and make sure that msharefs is following its
lead.  For instance, KVM _should_ have the exact same "page free"
flushing issue where it gets the MMU notifier call but the page may
still be in the secondary MMU.  I _think_ KVM fixes it with an extra
page refcount that it takes when it first walks the primary page tables.

But the short of it is that the msharefs host mm represents a "secondary
MMU".  I don't think it is really that special of an MMU other than the
fact that it has an mm_struct.