linux-kernel - Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu

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Message-ID: <bb1fcf44-09ca-73b1-5bbc-49f8bc51c8c1@redhat.com>
Date:   Wed, 27 Jan 2021 13:40:22 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Peter Xu <peterx@...hat.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu_pages with a lock

On 26/01/21 23:02, Sean Christopherson wrote:
>> You can do the deferred freeing with a short write-side critical section to
>> ensure all readers have terminated.
>
> Hmm, the most obvious downside I see is that the zap_collapsible_sptes() case
> will not scale as well as the RCU approach.  E.g. the lock may be heavily
> contested when refaulting all of guest memory to (re)install huge pages after a
> failed migration.

The simplest solution is to use a write_trylock on the read_unlock() 
path; if it fails, schedule a delayed work item 1 second in the future 
so that it's possible to do some batching.

(The work item would also have to re-check the llist after each iteration.)

Paolo