linux-kernel - Re: [PATCH RFC] pram: persistent over-kexec memory file system

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Date:	Sat, 27 Jul 2013 19:37:52 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, criu@...nvz.org, devel@...nvz.org,
	xemul@...allels.com
Subject: Re: [PATCH RFC] pram: persistent over-kexec memory file system

Il 27/07/2013 19:35, Vladimir Davydov ha scritto:
> On 07/27/2013 07:41 PM, Marco Stornelli wrote:
>> Il 26/07/2013 14:29, Vladimir Davydov ha scritto:
>>> Hi,
>>>
>>> We want to propose a way to upgrade a kernel on a machine without
>>> restarting all the user-space services. This is to be done with CRIU
>>> project, but we need help from the kernel to preserve some data in
>>> memory while doing kexec.
>>>
>>> The key point of our implementation is leaving process memory in-place
>>> during reboot. This should eliminate most io operations the services
>>> would produce during initialization. To achieve this, we have
>>> implemented a pseudo file system that preserves its content during
>>> kexec. We propose saving CRIU dump files to this file system, kexec'ing
>>> and then restoring the processes in the newly booted kernel.
>>>
>>
>> http://pramfs.sourceforge.net/
>
> AFAIU it's a bit different thing: PRAMFS as well as pstore, which has
> already been merged, requires hardware support for over-reboot
> persistency, so called non-volatile RAM, i.e. RAM which is not directly
> accessible and so is not used by the kernel. On the contrary, what we'd
> like to have is preserving usual RAM on kexec. It is possible, because
> RAM is not reset during kexec. This would allow leaving applications
> working set as well as filesystem caches in place, speeding the reboot
> process as a whole and reducing the downtime significantly.
>
> Thanks.

Actually not. You can use normal system RAM reserved at boot with mem 
parameter without any kernel change. Until an hard reset happens, that 
area will be "persistent".

Regards,

Marco
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