linux-kernel - Re: [PATCH v3] KVM: x86/mmu: Retry fault before acquiring mmu

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Message-ID: <b9f6e379-f517-4c24-8df4-1e5484515324@proxmox.com>
Date: Tue, 6 Feb 2024 14:52:33 +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>
Subject: Re: [PATCH v3] KVM: x86/mmu: Retry fault before acquiring mmu_lock if
 mapping is changing

On 03/02/2024 01:35, Sean Christopherson wrote:
> Retry page faults without acquiring mmu_lock if the resolved gfn is covered
> by an active invalidation.  Contending for mmu_lock is especially
> problematic on preemptible kernels as the mmu_notifier invalidation task
> will yield mmu_lock (see rwlock_needbreak()), delay the in-progress
> invalidation, and ultimately increase the latency of resolving the page
> fault.  And in the worst case scenario, yielding will be accompanied by a
> remote TLB flush, e.g. if the invalidation covers a large range of memory
> and vCPUs are accessing addresses that were already zapped.
[...]

Can confirm this patch fixes temporary guest hangs in combination with
KSM and NUMA balancing I'm seeing [1], which is likely to be the same
issue as described in [2]:

* On this patch's base-commit 60eedcfc from
git.kernel.org/pub/scm/virt/kvm/kvm.git, I can reproduce the hangs (see
[1] for reproducer)
* With this patch applied on top, I cannot reproduce the hangs anymore.

Thanks!

[1]
https://lore.kernel.org/kvm/832697b9-3652-422d-a019-8c0574a188ac@proxmox.com/
[2] https://bugzilla.kernel.org/show_bug.cgi?id=218259