linux-kernel - Re: Possible broken MM code in dell-laptop.c?

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Message-ID: <20150617034334.GB29788@vmdeb7>
Date:	Tue, 16 Jun 2015 20:43:34 -0700
From:	Darren Hart <dvhart@...radead.org>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Michal Hocko <mhocko@...e.cz>, Hans de Goede <hdegoede@...hat.com>,
	Ben Skeggs <bskeggs@...hat.com>,
	Stuart Hayes <stuart_hayes@...l.com>,
	Matthew Garrett <mjg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	platform-driver-x86@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: Possible broken MM code in dell-laptop.c?

On Tue, Jun 16, 2015 at 09:15:23AM +0200, Pali Rohár wrote:
> On Tuesday 16 June 2015 08:33:46 Michal Hocko wrote:
> > On Mon 15-06-15 23:27:59, Pali Rohár wrote:
> > > On Monday 15 June 2015 23:18:16 Michal Hocko wrote:
> > > > On Sun 14-06-15 11:05:07, Pali Rohár wrote:
> > > > > Hello,
> > > > > 
> > > > > in drivers/platform/x86/dell-laptop.c is this part of code:
> > > > > 
> > > > > static int __init dell_init(void)
> > > > > {
> > > > > ...
> > > > > 
> > > > > 	/*
> > > > > 	
> > > > > 	 * Allocate buffer below 4GB for SMI data--only 32-bit physical
> > > > > 	 addr * is passed to SMI handler.
> > > > > 	 */
> > > > > 	
> > > > > 	bufferpage = alloc_page(GFP_KERNEL | GFP_DMA32);
> > > > 
> > > > [...]
> > > > 
> > > > > 	buffer = page_address(bufferpage);
> > > > 
> > > > [...]
> > > > 
> > > > > fail_rfkill:
> > > > > 	free_page((unsigned long)bufferpage);
> > > > 
> > > > This one should be __free_page because it consumes struct page* and
> > > > it is the proper counter part for alloc_page. free_page, just to
> > > > make it confusing, consumes an address which has to be translated to
> > > > a struct page.
> > > > 
> > > > I have no idea why the API has been done this way and yeah, it is
> > > > really confusing.
> > > > 
> > > > [...]
> > > > 
> > > > > static void __exit dell_exit(void)
> > > > > {
> > > > > ...
> > > > > 
> > > > > 	free_page((unsigned long)buffer);
> > > 
> > > So both, either:
> > > 
> > >  free_page((unsigned long)buffer);
> > > 
> > > or
> > > 
> > >  __free_page(bufferpage);
> > > 
> > > is correct?
> > 
> > Yes. Although I would use __free_page variant as both seem to be
> > globally visible.
> > 

Michal - thanks for the context.

I'm surprised by your recommendation to use __free_page() out here in platform
driver land.

I'd also prefer that the driver consistently free the same address to avoid
confusion.

For these reasons, free_page((unsigned long)buffer) seems like the better
option.

Can you elaborate on why you feel __free_page() is a better choice?

-- 
Darren Hart
Intel Open Source Technology Center
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