linux-kernel - Re: [PATCH v4 0/4] KVM: x86/mmu: Pre-check for mmu

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Message-ID: <19ac27b1-1da1-4b31-a0b2-c25574515ac5@proxmox.com>
Date: Wed, 14 Feb 2024 14:17:19 +0100
From: Friedrich Weber <f.weber@...xmox.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Yan Zhao <yan.y.zhao@...el.com>, Kai Huang <kai.huang@...el.com>,
 Yuan Yao <yuan.yao@...ux.intel.com>, Xu Yilun <yilun.xu@...ux.intel.com>,
 Yu Zhang <yu.c.zhang@...ux.intel.com>,
 Chao Peng <chao.p.peng@...ux.intel.com>, Fuad Tabba <tabba@...gle.com>,
 Michael Roth <michael.roth@....com>,
 Isaku Yamahata <isaku.yamahata@...el.com>,
 David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v4 0/4] KVM: x86/mmu: Pre-check for mmu_notifier retry

On 09/02/2024 23:28, Sean Christopherson wrote:
> Retry page faults without acquiring mmu_lock, and potentially even without
> resolving a pfn, if the gfn is covered by an active invalidation.  This
> avoids resource and lock contention, which can be especially beneficial
> for preemptible kernels as KVM can get stuck bouncing mmu_lock between a
> vCPU and the invalidation task the vCPU is waiting on to finish.
> 
> v4: 
>  - Pre-check for retry before resolving the pfn, too. [Yan]
>  - Add a patch to fix a private/shared vs. memslot validity check
>    priority inversion bug.
>  - Refactor kvm_faultin_pfn() to clean up the handling of noslot faults.

Can confirm that v4 also fixes the temporary guest hangs [1] I'm seeing
in combination with KSM and NUMA balancing:
* On 60eedcfc, the reproducer [1] triggers temporary hangs
* With the four patches applied on top of 60eedcfc, the reproducer does
not trigger hangs

Thanks a lot for looking into this!

[1]
https://lore.kernel.org/kvm/832697b9-3652-422d-a019-8c0574a188ac@proxmox.com/