linux-kernel - Re: [linux-pm] Re: Hibernation considerations

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Date:	Sat, 21 Jul 2007 00:19:39 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jeremy Maitin-Shepard <jbms@....edu>
Cc:	Milton Miller <miltonm@....com>, Ying Huang <ying.huang@...el.com>,
	Alan Stern <stern@...land.harvard.edu>,
	LKML <linux-kernel@...r.kernel.org>, David Lang <david@...g.hm>,
	linux-pm <linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] Re: Hibernation considerations

On Friday, 20 July 2007 23:33, Jeremy Maitin-Shepard wrote:
> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> 
> [snip]
> 
> >> Or add a small bit of infrastructure that errors writes at make_request 
> >> if you don't have a magic "i am a direct block device write from 
> >> userspace" flag on the bio.
> >> 
> >> The hibernate may fail, but you don't corrupt the media.
> >> 
> >> If you don't get the image out, resume back to the "this is resume" 
> >> instead of the power-down path.
> 
> > Well, I don't think that is much prettier than the freezer ...
> 
> It seems that a better solution to the "how do we write to a file on an
> in-use partition" has been suggested, which also handles swap partitions
> and swap files, and does not require mounting filesystems, so it seems
> that the filesystem issue need not be considered.
> 
> [snip]
> 
> > No.  I'm saying that when you go back from the image-saving kernel to the
> > hibernated kernel, you need to make sure that no task will cause any
> > filesystem's on-disk state to be actually updated.  If you can't make such
> > a guarantee, you just can't do that.
> 
> > With the current state of the drivers, it's not doable without the
> > freezer.
> 
> It seems that it should be feasible to fix the drivers so that
> 
> 1. they can be taken from normal state to quiesced state without
>    requiring the freezer;
> 
> 2. they can be taken from normal state to low power state without
>    requiring the freezer;

Yes, that's correct.

> 3. they can be taken from quiesced state to low power state without
>    requiring the freezer.
>
> In the particular, it seems that it should be possible to do (3) without
> needing to schedule tasks.

For that, you'd have to forbid the drivers to call schedule() from the relevant
callbacks, which means, eg. no timeouts in there.

> It seems likely that (2) may in fact be almost exactly the same as, or
> at least similar to, (1) followed by (3), at least for many drivers.
> (1) is required by the kexec hibernate approach even ignoring suspend to
> both or S4.  (2) is required for suspend to ram without the freezer,
> which seems to be desired anyway.

Yes, (2) is needed anyway.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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