Message-ID: <87wshxkt9c.fsf@denkblock.local>
Date:	Sun, 31 Aug 2008 19:07:43 +0200
From:	Elias Oltmanns <eo@...ensachen.de>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
Cc:	Tejun Heo <htejun@...il.com>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Jeff Garzik <jeff@...zik.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] libata: Implement disk shock protection support

Bartlomiej Zolnierkiewicz <bzolnier@...il.com> wrote:
> Hi,
>
> On Sunday 31 August 2008, Tejun Heo wrote:
>> Hello,
>> 
>> Elias Oltmanns wrote:
>> >> Ah.. you need to part ATAPI too?  If so, just test for
>> >> ata_dev_enabled().
>> > 
>> > Well, I'm not quite sure really. Perhaps you are right and I'd better
>> > leave ATAPI alone, especially given the problem that the unload command
>> > might mess up a CD/DVD write operation. As long as no laptop HDs are
>> > identified as ATAPI devices, there should be no problem with that.
>> 
>> Hmm... I think it would be safer to stick with ATA for the time being.
>
> Seconded.  To be honest I also don't like the change to issue UNLOAD to
> all devices on the port (it only needlessly increases complexity right now
> since the _only_ use case at the moment is ThinkPad w/ hdaps + 1 HD).

Admittedly, I don't know very much about it myself but I seem to
remember that there are other vendors now shipping similar technology.
Even in the Thinkpad case I don't *know* that all relevant models have
HD and optical drives on seperate ports although I'm willing to believe
it if somebody tells me so.

Anyway, I've added Henrique to the Cc: list since he knows far more
about Thinkpads than I do and possibly about other notebooks too.

>
> [ I really hoped for the minimal initial implementation. ]

There is a lot to be said for the per-port solution as far as libata is
concerned. For the sake of consistency I tried to mimic the same
behaviour in ide but I agree that it makes things more complex there.

>
>> >> Can you please elaborate a bit?  The reloading is done by the kernel
>> >> after a timeout, right?  What kind of precarious situation can the
>> >> kernel get into regarding suspend?
>> > 
>> > Sorry, I haven't expressed myself very clearly there, it seems. The user
>> > space process detects some acceleration and starts writing timeouts to
>> > the sysfs file. This causes the unload command to be issued to the
>> > device and stops all I/O until the user space daemon decides that the
>> > danger has passed, writes 0 to the sysfs file and leaves it alone
>> > afterwards. Now, if the daemon happens to request head parking right at
>> > the beginning of a suspend sequence, this means that we are in danger of
>> > falling, i.e. we have to make sure that I/O is stopped until that user
>> > space daemon gives the all-clear. However, suspending implies freezing
>> > all processes which means that the daemon can't keep checking and
>> > signalling to the kernel. The last timeout received before the daemon
>> > has been frozen will expire and the suspend procedure goes ahead. By
>> > means of the notifiers we can make sure that suspend is blocked until
>> > the daemon says that everything is alright.
>> 
>> Is it really worth protecting against that?  What if the machine
>> started to fall after the userland tasks have been frozen?  And how
>> long the usual timeout would be?  If the machine has been falling for
>> 10 secs, there really isn't much point in trying to protect anything
>> unless there also is ATA DEPLOY PARACHUTE command somewhere in the new
>> revision.
>> 
>> In libata, as with any other exceptions, suspend/resume are handled by
>> EH so while emergency head unload is in progress, suspend won't
>> commence which is about the same effect as the posted code sans the
>> timeout extension part.  I don't really think there's any significant
>> danger in not being able to extend timeout while suspend is in
>> progress.  It's not a very big window after all.  If you're really
>> worried about it, you can also let libata reject suspend if head
>> unload is in progress.
>> 
>> Also, the suspend operation is unloading the head and spin down the
>> drive which sound like a good thing to do before crashing.  Maybe we
>> can modify the suspend sequence such that it always unload the head
>> first and then issue spindown.  That will ensure the head is in safe
>> position as soon as possible.  If it's done this way, it'll be
>> probably a good idea to split unloading and loading operations and do
>> loading only when EH is being finished and the disk is not spun down.
>> 
>> To me, much more serious problem seems to be during hibernation.  The
>> kernel is actively writing memory image to userland and it takes quite
>> a while and there's no protection whatsoever during that time.
>
> Which also brings again the question whether it is really the best to
> use user-space solution instead of kernel thread?
>
> After taking the look into the deamon program and hdaps driver I tend
> to "Nope." answer.  The kernel-space solution would be more reliable,
> should result in significatly less code and would free us from having
> a special purpose libata/ide interfaces.  It should also make the
> maintainance and future enhancements (i.e. making hibernation unload
> friendly) a lot easier.
>
> I imagine that this comes a bit late but can we at least give it an
> another thought, please?

Right, I'll try to give a concise statement of the problem. First
though, I absolutely agree that with regard to the suspend / hibernate
problem, an in kernel solution would ultimately be the safest option.
However, the way I see it, we would need a module with the following
characteristics:

- Policy: logic to decide when to park / unpark disks and an interface
  to export tunables to user space.
- Input: capability to recognise and register with acceleration sensors
  in the system and to gather data in an efficient manner. Since this is
  kernel space, we have to make it bulletproof and account for the
  possibility that there may be more than one such sensor installed in
  the system (think: plug and play).
- Action: find all rotating media in the system and decide which of them
  to protect how. Probably, some tunables for the user to fiddle with
  are required here too. Remember that we have docking stations and the
  like so more than one HD may show up on the bus.

All these corner cases that most users don't care or even tink about
won't hurt anyone as long as the daemon is in user space. This way, we
have a very simple solution for all of them: The user decides for each
instance of the daemon which accelerometer it gets its data from and
which HD it is supposed to protect. I don't like giving impressively
high percentages when all I'm doing is intelligent guess work, but the
vast majority of users will have only one daemon running, getting its
data from one accelerometer and protecting exactly one HD. However, it
is hard to imagine anything disastrous to happen *if* somebody should
happen to install a second accelerometer or connect to the docking
station. In kernel space we would have to take care of the oddest things
because a system supposed to increase security would suffer under a
reputation of locking the machine for good.

Regards,

Elias
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