linux-kernel - Re: [PATCH v12 84/84] KVM: Don't grab reference on VM

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Message-ID: <57ba7e1d-0121-4d71-89b8-c61c476ca724@redhat.com>
Date: Wed, 31 Jul 2024 11:50:38 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
 Huacai Chen <chenhuacai@...nel.org>, Michael Ellerman <mpe@...erman.id.au>,
 Anup Patel <anup@...infault.org>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 David Matlack <dmatlack@...gle.com>, David Stevens <stevensd@...omium.org>
Subject: Re: [PATCH v12 84/84] KVM: Don't grab reference on VM_MIXEDMAP pfns
 that have a "struct page"

On 7/30/24 22:21, Sean Christopherson wrote:
> On Tue, Jul 30, 2024, Paolo Bonzini wrote:
>> On 7/27/24 01:52, Sean Christopherson wrote:
>>> Now that KVM no longer relies on an ugly heuristic to find its struct page
>>> references, i.e. now that KVM can't get false positives on VM_MIXEDMAP
>>> pfns, remove KVM's hack to elevate the refcount for pfns that happen to
>>> have a valid struct page.  In addition to removing a long-standing wart
>>> in KVM, this allows KVM to map non-refcounted struct page memory into the
>>> guest, e.g. for exposing GPU TTM buffers to KVM guests.
>>
>> Feel free to leave it to me for later, but there are more cleanups that
>> can be made, given how simple kvm_resolve_pfn() is now:
> 
> I'll revisit kvm_resolve_pfn(), Maxim also wasn't a fan of a similar helper that
> existed in v11.

FWIW kvm_resolve_pfn() is totally fine as an intermediate step.  Just 
food for thought for possible follow-ups.

>> Also, check_user_page_hwpoison() should not be needed anymore, probably
>> not since commit 234b239bea39 ("kvm: Faults which trigger IO release the
>> mmap_sem", 2014-09-24) removed get_user_pages_fast() from hva_to_pfn_slow().
> 
> Ha, I *knew* this sounded familiar.  Past me apparently came to the same
> conclusion[*], though I wrongly suspected a memory leak and promptly forgot to
> ever send a patch.  I'll tack one on this time around.

As you prefer.

Paolo