Message-ID: <20140209093032.GM17045@pengutronix.de>
Date:	Sun, 9 Feb 2014 10:30:32 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Yann Droneaud <ydroneaud@...eya.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [PATCHv4] driver core/platform: don't leak memory allocated for
 dma_mask

Hello Yann,

On Sun, Feb 09, 2014 at 08:47:22AM +0100, Yann Droneaud wrote:
> Hi,
> 
> Le samedi 08 février 2014 à 16:09 +0100, Uwe Kleine-König a écrit :
> > On Fri, Feb 07, 2014 at 03:20:05PM -0800, Greg Kroah-Hartman wrote:
> > > On Mon, Jan 27, 2014 at 11:05:52AM +0100, Yann Droneaud wrote:
> > > > Since commit 01dcc60a7cb8, platform_device_register_full() is
> > > > available to allocate and register a platform device.
> > > > 
> > > > If a dma_mask is provided as part of platform_device_info,
> > > > platform_device_register_full() allocate memory for a u64 using
> > > > kmalloc().
> > > > 
> > > > A comment in the code state that "[t]his memory isn't freed when the
> > > > device is put".
> > > > 
> > > > It's never a good thing to leak memory, but there's only very few
> > > > users of platform_device_info's dma_mask, and those are mostly
> > > > "static" devices that are not going to be plugged/unplugged often.
> > > > 
> > > > So memory leak is not really an issue, but allocating 8 bytes through
> > > > kmalloc() seems overkill.
> > > > 
> > > > And it's a pity to have to allocate 8 bytes for the dma_mask while up
> > > > to 7 bytes are wasted at the end of struct platform_object in the form
> > > > of padding after name field: unfortunately this padding is not used
> > > > when allocating the memory to hold the name.
> > > >
> > > > To address theses issues, this patch adds dma_mask to platform_object
> > > > struct so that it is always allocated for all struct platform_device
> > > > allocated through platform_device_alloc() or platform_device_register_full().
> > > > And since it's within struct platform_object, dma_mask won't be leaked
> > > > anymore when struct platform_device got released. Storage for dma_mask
> > > > is added almost for free by removing the padding at the end of struct
> > > > platform_object.
> 
> > How does the padding of name change? The only thing that I see changing
> > for .name is that it's a char[] now instead of a char[1]. As it was
> > used as a flexible array already before the padding (which only applies
> > to a stand alone struct platform_object) doesn't matter.
> > I guess that is a tool problem that still some padding changes are
> > reported?
> > 
> 
> When name is defined as char name[1] at the end of the structure, the
> compiler is required to add padding after it (since the structure is not
> "packed" through some compiler extension).
> This padding is added in order to have a size multiple of the highest
> required alignement for types insided the structure. This is required so
> that each item of an array of "struct platform_object" are aligned.
> 
> Changing name[1] to name[0] or name[] free the compiler from adding
> storage space for name and thus remove the need for padding after it.
Ah, I see, even though .name was used as a flexible array there was too
much allocated because sizeof(struct platform_object) includes the 7
bytes of padding.

Maybe it makes sense to split the patch into a) s/name[1]/name[]/ and b)
add dma_mask? Up to you, you can have my Ack also for the single patch.

Acked-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

Thanks
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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