linux-kernel - Re: [PATCH] Remove process freezer from suspend to RAM pathway

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Message-Id: <200707051535.46196.rjw@sisk.pl>
Date:	Thu, 5 Jul 2007 15:35:45 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	nigel@...pend2.net
Cc:	Pavel Machek <pavel@....cz>, Oliver Neukum <oliver@...kum.org>,
	Miklos Szeredi <miklos@...redi.hu>, benh@...nel.crashing.org,
	mjg59@...f.ucam.org, linux-kernel@...r.kernel.org,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [PATCH] Remove process freezer from suspend to RAM pathway

On Thursday, 5 July 2007 14:38, Nigel Cunningham wrote:
> Hi.
> 
> On Thursday 05 July 2007 22:25:06 Rafael J. Wysocki wrote:
> > On Thursday, 5 July 2007 01:45, Pavel Machek wrote:
> > > On Tue 2007-07-03 21:32:20, Oliver Neukum wrote:
> > > > Am Dienstag, 3. Juli 2007 schrieb Miklos Szeredi:
> > > > > > And a further question. The freezer is not atomic. What do you do
> > > > > > if a task not yet frozen calls sys_sync(), but fuse is already 
> frozen?
> > > > > 
> > > > > What do you do if a task not yet frozen writes to a pipe, on the other
> > > > > end of which is a task already frozen?
> > > 
> > > There's some difference between uninterruptible and interruptible
> > > sleep I'd say.
> > > 
> > > > > It doesn't matter.  The only thing that should matter during suspend
> > > > > (not hibernate) is saving the state of devices to ram, and putting the
> > > > > devices to sleep.
> > > > 
> > > > Well, but you did remove sys_sync() from the freezer, which is
> > > > and must be called in the hibernate path.
> > > 
> > > Not "must". In fact, hibernation should be safe without sys_sync(). It
> > > is just user un-friendly.
> > 
> > In fact, I'd like to remove the sys_sync() from the freezer entirely, 
> because
> > it just doesn't belong in there.
> > 
> > The only advantege of having sys_sync() in freeze_processes() is that we
> > have a chance to write out everything when applications cannot produce more
> > data to write, but there are filesystems which don't do that anyway (eg. 
> XFS),
> > so generally there's no reason to bother.
> 
> Shouldn't XFS - and fuse - be considered to be broken? Sync should sync data 
> and if XFS isn't doing that, it's wrong.
> 
> In the case of fuse, we should have a mechanism by which fuse processes can be 
> made to sync if they do have any pending I/O, and by which they can be frozen 
> later than other userspace processes.
> 
> I'd like to see the sync stay, because it improves reliability and data 
> integrity in the fail-to-resume case. Calling scripts would probably invoke 
> sync themselves if they don't already, but that's racy. As it is at the 
> moment, we know userspace is stopped, so syncing isn't racy.

I'd like to move the sync out of the freezer, but to call it from the
suspend/hibernation code, so that we do

sys_sync();
error = freeze_processes();

etc.

Greetings,
Rafael


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