Date:	Thu, 03 Dec 2009 12:57:28 +0000
From:	Alan Jenkins <alan-jenkins@...fmail.co.uk>
To:	Pavel Machek <pavel@....cz>
CC:	Mel Gorman <mel@....ul.ie>, "Rafael J. Wysocki" <rjw@...k.pl>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [PATCH] uswsusp: automatically free the in-memory image once
 s2disk has finished with it

Pavel Machek wrote:
> On Wed 2009-12-02 22:25:16, Mel Gorman wrote:
>   
>> On Wed, Dec 02, 2009 at 11:15:24PM +0100, Pavel Machek wrote:
>>     
>>> On Wed 2009-12-02 22:07:18, Mel Gorman wrote:
>>>       
>>>> On Wed, Dec 02, 2009 at 10:11:07PM +0100, Pavel Machek wrote:
>>>>         
>>>>> On Wed 2009-12-02 14:28:12, Alan Jenkins wrote:
>>>>>           
>>>>>> The original in-kernel suspend (swsusp) frees the in-memory hibernation
>>>>>> image before powering off the machine.  s2disk doesn't, so there is
>>>>>> _much_ less free memory when it tries to power off.
>>>>>>
>>>>>> This is a gratuitous difference.  The userspace suspend interface
>>>>>> /dev/snapshot only allows the hibernation image to be read once.
>>>>>> Once the s2disk program has read the last page, we can free the entire
>>>>>> image.
>>>>>>
>>>>>> This avoids a hang after writing the hibernation image which was
>>>>>> triggered by commit 5f8dcc21211a3d4e3a7a5ca366b469fb88117f61
>>>>>> "page-allocator: split per-cpu list into one-list-per-migrate-type":
>>>>>>             
>>>>> Yes, you work around page-allocator hang. But is it right thing to do?
>>>>>
>>>>>           
>>>> What's wrong with it? The hang is likely because the allocator has no
>>>> memory to work with. The patch in question makes small changes to the
>>>> amount of available memory but it shouldn't matter on uni-core. Some
>>>> structures are slightly larger but it's extremely borderline. I'm at a
>>>> loss to explain actually why it makes a difference untill things were
>>>> extremely borderline to begin with.
>>>>         
>>> We reserve 4MB, for such purposes, and we already wrote image to disk
>>> with such constrains, so memory should not be _too_ tight.
>>>
>>> Can you try increasing PAGES_FOR_IO to 8MB or something like that?
>>>
>>>       
>> What's wrong with just freeing the memory that is no longer required?
>>     
>
> Nothing. But 4MB was enough to power down before, it is not enough
> now, and I'd like to understand why.
> 									Pavel
>   

Here's a new datum:

Applying this patch has left a less frequent hang.  So far it has 
happened twice.  (Once playing last night, and once today testing 
hibernation with KMS enabled).

This hang happens at a different point.  It happens _before_ writing out 
the hibernation image.  That is, I don't see the textual progress bar, 
and if I force a power-cycle then it doesn't resume (and complains about 
uncleanly unmounted filesystems).

Here is the backtrace:

[top of screen]
s2disk D c1c05580 0 5988 5809 0x00000000
...
Call Trace:
...
? wait_for_common
? default_wake_function
? kthread_create
? worker_thread
? create_workqueue_thread
? worker_thread
? __create_workqueue_thread
? stop_machine_create
? disable_nonboot_cpus
? hibernation_snapshot
? snapshot_ioctl
...
? sys_ioctl


It looks like hibernation_snapshot() calls disable_nonboot_cpus() 
_before_ we allocate the hibernation image.  (I.e. before 
swsusp_arch_suspend(), which calls swsusp_save()).

So I think Pavel's right, we still need to work out what's happening here.

Regards
Alan
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