linux-kernel - Re: [PATCH] KVM: x86/mmu: Update number of zapped pages even if page list is stable

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Date:   Thu, 11 Nov 2021 23:18:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: Update number of zapped pages even if page
 list is stable

On Thu, Nov 11, 2021, Ben Gardon wrote:
> On Thu, Nov 11, 2021 at 2:14 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > Fixes: fbb158cb88b6 ("KVM: x86/mmu: Revert "Revert "KVM: MMU: zap pages in batch""")
> > Reported-by: David Matlack <dmatlack@...gle.com>
> > Cc: Ben Gardon <bgardon@...gle.com>
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> 
> Reviewed-by: Ben Gardon <bgardon@...gle.com>
> 
> While I can see this fixing the above stall, there's still a potential
> issue where zapped_obsolete_pages can accumulate an arbitrary number
> of pages from multiple batches of zaps. If this list gets very large,
> we could see a stall after the loop while trying to free the pages.
> I'm not aware of this ever happening, but it could be worth yielding
> during that freeing process as well.

Ya.  I tagged this one for stable because its very much a regression that I
introduced when reverting the revert, i.e. the very original implemenation worked.
Sadly, I did not get to do a triple revert :-)