linux-kernel - Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in get_dirty

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Message-Id: <20120314100435.16c1bfcf.yoshikawa.takuya@oss.ntt.co.jp>
Date:	Wed, 14 Mar 2012 10:04:35 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	avi@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in
 get_dirty_log()

Marcelo Tosatti <mtosatti@...hat.com> wrote:

> 
> This is a race with hugetlbfs which is not an issue ATM (it is 
> hidden by the removal of huge sptes in get_dirty).

Thank you!
I did not notice this possibility at all.

...

> It can be fixed with a preceding patch that checks whether
> slot->dirty_bitmap value changes between mapping_level and after
> mmu_lock acquision, similarly to mmu_seq. Also please add a 
> WARN_ON in mmu_set_spte if(slot->dirty_bitmap && level > 1).
> And document it clearly.

We may also be able to change the "if (slot->dirty_bitmap)" check to
use another flag so that we can delay the start of logging until
mmu_lock acquisition in kvm_set_memory_region().

Looking dirty_bitmap directly should be limited to when we are in
get_dirty functions.

	Takuya
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