linux-kernel - Re: [Xen-devel] [PATCH] mmap_vmcore: skip non-ram pages reported by hypervisors

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Message-ID: <53BC1B96.3070101@citrix.com>
Date:	Tue, 8 Jul 2014 17:25:58 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Vitaly Kuznetsov <vkuznets@...hat.com>
CC:	<xen-devel@...ts.xenproject.org>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	<linux-kernel@...r.kernel.org>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [Xen-devel] [PATCH] mmap_vmcore: skip non-ram pages reported
 by hypervisors

On 07/07/14 21:33, Andrew Morton wrote:
> On Mon,  7 Jul 2014 17:05:49 +0200 Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
>> we have a special check in read_vmcore() handler to check if the page was
>> reported as ram or not by the hypervisor (pfn_is_ram()). However, when
>> vmcore is read with mmap() no such check is performed. That can lead to
>> unpredictable results, e.g. when running Xen PVHVM guest memcpy() after
>> mmap() on /proc/vmcore will hang processing HVMMEM_mmio_dm pages creating
>> enormous load in both DomU and Dom0.

Does make forward progress though?  Or is it ending up in a repeatedly
retrying the same instruction?

Is it failing on a ballooned page in a RAM region? Or is mapping non-RAM
regions as well?

>> Fix the issue by mapping each non-ram page to the zero page. Keep direct
>> path with remap_oldmem_pfn_range() to avoid looping through all pages on
>> bare metal.
>>
>> The issue can also be solved by overriding remap_oldmem_pfn_range() in
>> xen-specific code, as remap_oldmem_pfn_range() was been designed for.
>> That, however, would involve non-obvious xen code path for all x86 builds
>> with CONFIG_XEN_PVHVM=y and would prevent all other hypervisor-specific
>> code on x86 arch from doing the same override.

The oldmem_pfn_is_ram() is Xen-specific but this problem (ballooned
pages) must be common to KVM.  How does KVM handle this?

David
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