Re: [RFC][PATCH] PM: Force GFP_NOIO during suspend/resume (was: Re: [linux-pm] Memory allocations in .suspend became very unreliable)

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Hi 

> > If suspend need lots memory, we need to make free memory before starting IO
> > suspending, I think.
> 
> Suspend as such doesn't need a lot of memory, except for some drivers doing
> things they shouldn't do.
> 
> However, there are a few problems that need to be addressed in general.
> 
> First, we can't really guarantee that there's a lot of free memory available
> during suspend and some memory allocations are done indirectly, using
> GFP_KERNEL (for example, when new kernel threads are started).  If one of
> these is done during suspend and it happens to cause the mm subsystem to
> start I/O on a suspended devices, the kernel will lock up.
> 
> Second, there may be a memory allocation in progress when suspend is started
> that causes I/O to happen and races with the suspend process.  If the latter
> wins the race, the I/O may be attempted on a suspended device and the kernel
> will lock up.

I think the race happen itself is bad. memory and I/O subsystem can't solve such race
elegantly. These doesn't know enough suspend state knowlege. I think the practical 
solution is that higher level design prevent the race happen.


> My patch attempts to avoid these two problems as well as the problem with
> drivers using GFP_KERNEL allocations during suspend which I admit might be
> solved by reworking the drivers.

Agreed. In this case, only drivers change can solve the issue.



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