linux-kernel - Re: freeze vs freezer

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Date:	Tue, 24 Jun 2008 10:08:17 +0200
From:	Elias Oltmanns <eo@...ensachen.de>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	Pavel Machek <pavel@...e.cz>, Kyle Moffett <mrmacman_g4@....com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	David Chinner <dgc@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, xfs-masters@....sgi.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: freeze vs freezer

Henrique de Moraes Holschuh <hmh@....eng.br> wrote:
> On Mon, 23 Jun 2008, Pavel Machek wrote:
>> (replying to *very* old mail).
>
>> 
>> >>>> We wait until they can continue.
>> >>>
>> >>> So if I have a process blocked on an unavilable NFS mount, I can't
>> >>> suspend?
>> >>
>> >> That's correct, you can't.
>> >>
>> >> [And I know what you're going to say. ;-)]
>> >
>> > Why exactly does suspend/hibernation depend on "TASK_INTERRUPTIBLE" instead 
>> > of a zero preempt_count()?  Really what we should do is just iterate over 
>> > all of the actual physical devices and tell each one "Block new IO requests 
>> > preemptably, finish pending DMA, put the hardware in low-power mode, and 
>> > prepare for suspend/hibernate".  As long as each driver knows how to do 
>> > those simple things we can have an entirely consistent kernel image for 
>> > both suspend and for hibernation.
>> 
>> Patch would be welcome, actually. It turns out blocking new
>> IO-requests is not completely trivial.

Quite. But I'm not sure I see what this is all about yet. From the IDE
and SCSI subsystems I remember that they block all I/O from higher levels
once the suspend callbacks have been executed. I haven't made an effort
to understand the freezer (or indeed anything related to hibernation)
yet since I don't even use hibernation myself (only s2ram). Do you have
any suggestion where to start reading up on things so I can get an idea
what the issues are and what you would like IDE / SCSI / ... to do?

>
> Is this the same thing the per-device IO-queue-freeze patches for
>HDAPS also
> need to do?  If so, you may want to talk to Elias Oltmanns
> <eo@...ensachen.de> about it.  Added to CC.

Thanks for the heads up Henrique. Even though these issues seem to be
related up to a certain degree, there probably are some important
differences. When suspending a system, the emphasis is on leaving the
system in a consistent state (think of journalled file systems), whereas
disk shock protection is mainly concerned with stopping I/O as soon as
possible. As yet, I cannot possibly say to what extend these two
concepts can be reconciled in the sense of sharing some common code.

Regards,

Elias
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