Date:	Sat, 04 Jul 2015 03:03:46 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Dave Chinner <david@...morbit.com>
Cc:	Len Brown <lenb@...nel.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Alan Stern <stern@...land.harvard.edu>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH 1/1] suspend: delete sys_sync()

On Friday, July 03, 2015 11:42:50 AM Dave Chinner wrote:
> On Wed, Jul 01, 2015 at 11:07:29PM -0400, Len Brown wrote:
> > >> The _vast_ majority of systems using Linux suspend today are under
> > >> an Android user-space.  Android has no assumption that that suspend to
> > >> mem will necessarily stay suspended for a long time.
> > >
> > > Indeed, however your change was not android-specific, and it is not
> > > "comfortable" on x86-style hardware and usage patterns.
> > 
> > "comfortable on x86-style and usage patterns"?
> > If you mean "traditional" instead of "comfortable",
> > where "tradition" is based on 10-year old systems, then sure.
> 
> Even if this were true(*) we don't break things that currently work
> just because something different is "just around the corner". e.g.
> if you shut the lid on your laptop and it suspends to RAM, you can
> pull the USB drive out that you just copied stuff to and plug it
> into another machine and find all the data you copied there is
> present.
> 
> Remove the sync() from the freeze code, and this isn't guaranteed to
> work anymore. It is now dependent on userspace implementations for
> this to work, and we know what userspace developers will choose in
> this situation. i.e. fast and "works for me", not "safe for
> everyone".
> 
> (*) Which it clearly isn't true because, as this example shows, my
> shiny new laptop still has exactly the same data integrity
> requirements as the laptop I was using 10 years ago.
> 
> Just because there are lots of Android or Chrome out there it
> doesn't mean we can just ignore the requirements of everything
> else...
> 
> > > That said, as long as x86 will still try to safeguard my data during mem
> > > sleep/resume as it does today, I have no strong feelings about
> > > light/heavy-weight "mem" sleep being strictly a compile-time selectable
> > > thing, or a more flexible runtime-selectable behavior.
> > 
> > The observation here is that the kernel should not force every system
> > to sys_sync() on every suspend.  The only question is how to best
> > implement that.
> 
> No, your observation was that "sync is slow". Your *solution* is "we
> need to remove sync".

Not only slow, but pointless too.  The argument goes: "It is slow and
pointless and so it may be dropped."

Now, I can agree that it wasn't clearly demonstrated that the unconditional
sys_sync() in the suspend code path was pointless, but it also has never
been clearly shown why it is not pointless on systems that suspend and resume
reliably.

[The argument that the user can pull removable storage devices out of the
system while suspended doesn't hold any water to me, because the user can
pull them out of the system when not suspended just as well and cause the
same kind of damage to happen.]

When we were adding it, the thinking was along the lines of "Well, suspend
isn't too reliable, so let's put sys_sync() in there to possibly reduce the
damage from suspend/resume crashes and suspend is slow anyway, so the possible
effect on performance from that shouldn't be noticeable".  Clearly, the world
has changed since then and suspend is far more reliable than it used to be in
general and it is not that slow too at least on some systems (especially the
suspend-to-idle flavor).

The only argument against dropping sys_sync() from the suspend code path
I've seen in this thread that I entirely agree with is that it may lead to
regressions, because we've done it practically forever and it may hide latent
bugs somewhere in block drivers etc.  Dropping it, though, is the only way
to see those bugs, if any, and if we want to ever fix them, we need to see
them.  That's why I think that it may be a good idea to allow people to
drop it if they are willing to accept some extra risk (via the kernel
command line, for example).

Moreover, question is if we really need to carry out the sync on *every*
suspend even if it is not pointless overall.  That shouldn't really be
necessary if we suspend and resume often enough or if we resume only for
a while and then suspend again.  Maybe it should be rate limited somehow
at least?

Thanks,
Rafael

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