linux-kernel - Re: [PATCH] KVM: nVMX: Don't return error on nested bitmap memory allocation failure

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Message-ID: <5541095C.8050305@redhat.com>
Date:	Wed, 29 Apr 2015 18:39:56 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>
CC:	Jan Kiszka <jan.kiszka@...mens.com>, kvm@...r.kernel.org,
	Wincy Van <fanwenyi0529@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: Don't return error on nested bitmap memory
 allocation failure



On 29/04/2015 18:08, Bandan Das wrote:
>>>> >> > Yeah... I hear you. Ok, let me put it this way - Assume that we can
>>>> >> > defer this allocation up until the point that the nested subsystem is
>>>> >> > actually used i.e L1 tries running a guest and we try to allocate this
>>>> >> > area. If get_free_page() failed in that case, would we still want to
>>>> >> > kill L1 too ? I guess no.
>>> >>
>>> >> We could block the hypervisor thread on the allocation, just like it
>>> >> would block on faults for swapped out pages or new ones that have to be
>>> >> reclaimed from the page cache first.
> So, block on a failure hoping that eventually it will succeed ?
> 
>> > In that case we should avoid making the allocation GFP_ATOMIC to begin with.
>> >
>> > If a GFP_KERNEL allocation failed, returning -ENOMEM from KVM_RUN (which
>> > practically means killing the guest) would actually be a very real
>> > possibility.
> Sorry Paolo, I missed your point. Isn't the allocation already GFP_KERNEL ?

I mean if it were done lazily as in your thought-experiment.  Then:

- a GFP_ATOMIC allocation would be bad

- a GFP_KERNEL allocation would block like Jan said; if it failed, I
would be okay with returning -ENOMEM to userspace, even if that in
practice means killing the guest.

Paolo
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