linux-kernel - Re: [PATCH 4/8] fbdev: ssd1307fb: Use vmalloc to allocate video memory.

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Message-ID: <20150214153617.GB25269@lukather>
Date:	Sat, 14 Feb 2015 16:36:17 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Thomas Niederprüm <niederp@...sik.uni-kl.de>
Cc:	linux-fbdev@...r.kernel.org, plagnioj@...osoft.com,
	tomi.valkeinen@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] fbdev: ssd1307fb: Use vmalloc to allocate video
 memory.

On Sat, Feb 14, 2015 at 03:22:12PM +0100, Thomas Niederprüm wrote:
> Am Thu, 12 Feb 2015 16:11:21 +0100
> schrieb Maxime Ripard <maxime.ripard@...e-electrons.com>:
> 
> > On Sat, Feb 07, 2015 at 04:35:41PM +0100, Thomas Niederprüm wrote:
> > > Am Sat, 7 Feb 2015 12:18:21 +0100
> > > schrieb Maxime Ripard <maxime.ripard@...e-electrons.com>:
> > > 
> > > > Hi,
> > > > 
> > > > On Fri, Feb 06, 2015 at 11:28:10PM +0100, niederp@...sik.uni-kl.de
> > > > wrote:
> > > > > From: Thomas Niederprüm <niederp@...sik.uni-kl.de>
> > > > > 
> > > > > It makes sense to use vmalloc to allocate the video buffer
> > > > > since it has to be page aligned memory for using it with mmap.
> > > > 
> > > > Please wrap your commit log at 80 chars.
> > > 
> > > I'll try to do so in future, sorry for that.
> > > 
> > > > 
> > > > It looks like there's numerous fbdev drivers using this
> > > > (especially since you copy pasted that code, without mentionning
> > > > it).
> > > 
> > > Yes, I should have mentioned that in the commit message. As
> > > implicitly indicated in the cover letter the rvmalloc() and
> > > rvfree() are copy pasted from the vfb driver. Honestly, I didn't
> > > give this one too much thought. It seemed a viable solution to the
> > > mmap problem. For a bit more history on that, see my comment below.
> > > 
> > > > 
> > > > That should be turned into an allocator so that drivers all get
> > > > this right.
> > > > 
> > > > > Also deffered io seems buggy in combination with kmalloc'ed
> > > > > memory (crash on unloading the module).
> > > > 
> > > > And maybe that's the real issue to fix.
> > > 
> > > The problem is solved by using vmalloc ;)
> > 
> > Yep, but why do you need to mark the reserved pages?
> > 
> > ...
> 
> As far as I understood mmaped memory is marked as userspace memory in
> the page table and is therefore subject to swapping. The pages are
> marked reserved to make clear that this memory can not be swapped and
> thus lock the pages in memory. See discussions [0,1,2]. 
> 
> [0]http://stackoverflow.com/questions/10760479/mmap-kernel-buffer-to-user-space
> [1]http://www.linuxquestions.org/questions/linux-kernel-70/why-why-setpagereserved-is-needed-when-map-a-kernel-space-to-user-space-885176/
> [2]https://sites.google.com/site/skartikeyan/mmap.html

Hmmm, both https://lwn.net/Articles/28746/ and
http://linux-mm.org/DeviceDriverMmap tell otherwise :)

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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