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

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Date:	Wed, 29 Apr 2015 15:05:57 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Bandan Das <bsd@...hat.com>
CC:	kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	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

Am 2015-04-29 um 14:55 schrieb Bandan Das:
> Jan Kiszka <jan.kiszka@...mens.com> writes:
> 
>> Am 2015-04-28 um 21:55 schrieb Bandan Das:
>>>
>>> If get_free_page() fails for nested bitmap area, it's evident that
>>> we are gonna get screwed anyway but returning failure because we failed
>>> allocating memory for a nested structure seems like an unnecessary big
>>> hammer. Also, save the call for later; after we are done with other
>>> non-nested allocations.
>>
>> Frankly, I prefer failures over automatic degradations. And, as you
>> noted, the whole system will probably explode anyway if allocation of a
>> single page already fails. So what does this buy us?
> 
> 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.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
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