linux-kernel - Re: [PATCH v2 3/3] PM / hibernate: Recycle safe pages after image restoration

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Date:	Tue, 16 Aug 2016 01:16:11 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Pavel Machek <pavel@....cz>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] PM / hibernate: Recycle safe pages after image restoration

On Monday, August 15, 2016 04:33:49 PM Pavel Machek wrote:
> On Thu 2016-08-11 23:23:20, Rafael J. Wysocki wrote:
> > On Thursday, August 11, 2016 11:06:15 PM Pavel Machek wrote:
> > > Hi!
> > > 
> > > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > 
> > > > One of the memory bitmaps used by the hibernation image restoration
> > > > code is freed after the image has been loaded.
> > > > 
> > > > That is not quite efficient, though, because the memory pages used
> > > > for building that bitmap are known to be safe (ie. they were not
> > > > used by the image kernel before hibernation) and the arch-specific
> > > > code finalizing the image restoration may need them.  In that case
> > > > it needs to allocate those pages again via the memory management
> > > > subsystem, check if they are really safe again by consulting the
> > > > other bitmaps and so on.
> > > > 
> > > > To avoid that, recycle those pages by putting them into the global
> > > > list of known safe pages so that they can be given to the arch code
> > > > right away when necessary.
> > > 
> > > Ok, so you are trying to gain speed here? How much is the speedup?
> > 
> > This is more about making it easier to debug than about speed, TBH.
> > 
> > Avoiding bitmap operations and the mm subsystem involvement reduces
> > complexity and the number of places to look into in case something goes
> > wrong.
> 
> Well, it looked like 3/3 just added code and did not remove anything,
> so I fail to see how it makes code easier to follow...

With the patch what it takes to release a safe page and re-use it is to
(a) put it into the list of safe pages and (b) take it back from that list
when needed.

Without it what that takes is to (a) clear bitmap bits corresponding to the
page in two bitmaps, (b) free it, (c) allocate it again (which may involve
allocating a number of other pages that aren't "safe") and (d) set bits
corresponding to it in the bitmaps.  Clearly, more complicated.

And now if there's a memory corruption bug that looks like it might be
related to that code, in the "with the patch" case we know that the bitmaps
were not involved, for example.

Thanks,
Rafael