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

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Date:	Fri, 22 Jan 2010 21:58:46 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-pm@...ts.linux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH] PM: Force GFP_NOIO during suspend/resume (was: Re: [linux-pm] Memory allocations in .suspend became very unreliable)

On Friday 22 January 2010, KOSAKI Motohiro wrote:
> > > Probably we have multiple option. but I don't think GFP_NOIO is good
> > > option. It assume the system have lots non-dirty cache memory and it isn't
> > > guranteed.
> > 
> > Basically nothing is guaranteed in this case.  However, does it actually make
> > things _worse_?  
> 
> Hmm..
> Do you mean we don't need to prevent accidental suspend failure?
> Perhaps, I did misunderstand your intention. If you think your patch solve
> this this issue, I still disagree.

No, I don't.

> but If you think your patch mitigate the pain of this issue, I agree it.

That's what I wanted to say really.

> I don't have any reason to oppose your first patch.

Great!

> > What _exactly_ does happen without the $subject patch if the
> > system doesn't have non-dirty cache memory and someone makes a GFP_KERNEL
> > allocation during suspend?
> 
> Page allocator prefer to spent lots time for reclaimable memory searching than
> returning NULL. IOW, it can spent time few second if it doesn't have
> reclaimable memory.
> In typical case, OOM killer forcely make enough free memory if the system
> don't have any memory. But under suspending time, oom killer is disabled.
> So, if the caller (probably drivers) call alloc >1000times, the system
> spent lots seconds.
> 
> In this case, GFP_NOIO doesn't help. slowness behavior is caused by
> freeable memory search, not slow i/o.
> 
> However, if strange i/o device makes any i/o slowness, GFP_NOIO might help.
> In this case, please don't ask me about i/o thing. I don't know ;)

OK, thanks for the explanation.

Rafael
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