linux-kernel - Re: [PATCH v13 21/21] KVM: pfncache: rework __kvm_gpc

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Message-ID: <Zc-EwMoijOo7w49N@google.com>
Date: Fri, 16 Feb 2024 07:52:32 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: paul@....org
Cc: David Woodhouse <dwmw2@...radead.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Jonathan Corbet <corbet@....net>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, 
	David Hildenbrand <david@...hat.com>, Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-s390@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v13 21/21] KVM: pfncache: rework __kvm_gpc_refresh() to
 fix locking issues

On Fri, Feb 16, 2024, Paul Durrant wrote:
> On 16/02/2024 13:04, David Woodhouse wrote:
> > On Thu, 2024-02-15 at 15:29 +0000, Paul Durrant wrote:
> > > From: David Woodhouse <dwmw@...zon.co.uk>
> > > 
> > > This function can race with kvm_gpc_deactivate(), which does not take
> > > the ->refresh_lock. This means kvm_gpc_deactivate() can wipe the ->pfn
> > > and ->khva fields, and unmap the latter, while hva_to_pfn_retry() has
> > > temporarily dropped its write lock on gpc->lock.
> > 
> > Let's drop this from your series for now, as it's contentious.
> > 
> > Sean didn't like calling it a 'fix', which I had conceded and reworked
> > the commit message. It was on the list somewhere, and also in
> > https://git.infradead.org/users/dwmw2/linux.git/commitdiff/f19755000a7
> > 
> > I *also* think we should do this simpler one:
> > https://git.infradead.org/users/dwmw2/linux.git/commitdiff/cc69506d19a
> > ... which almost makes the first one unnecessary, but I think we should
> > do it *anyway* because the rwlock abuse it fixes is kind of awful.
> > 
> > And while we still can't actually *identify* the race condition that
> > led to a dereference of a NULL gpc->khva while holding the read lock
> > and gpc->valid and gpc->active both being true... I'll eat my hat if
> > cleaning up and simplifying the locking (and making it self-contained)
> > *doesn't* fix it.

Heh, I'm not taking that bet.

> > But either way, it isn't really part of your series. The only reason it
> > was tacked on the end was because it would have merge conflicts with
> > your series, which had been outstanding for months already.
> > 
> > So drop this one, and I'll work this bit out with Sean afterwards.

FWIW, I'm not opposed to overhauling the gpc locking, I agree it's a mess.  I just
want to proceed slower than I would for a fix, it's a lot to digest.

> Ok. Sean, I assume that since this is the last patch in the series it's
> superfluous for me to post a v14 just for this?

Correct, definitely no need for a new version.