Re: [PATCH v2 4/6] devres: handle zero size in devm_kmalloc()

From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Jonathan Corbet <corbet@lwn.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	Guenter Roeck <linux@roeck-us.net>,
	Jean Delvare <jdelvare@suse.com>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	linux-hwmon@vger.kernel.org,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>
Subject: Re: [PATCH v2 4/6] devres: handle zero size in devm_kmalloc()
Date: Sat, 10 Apr 2021 20:21:24 -0700	[thread overview]
Message-ID: <CAKdAkRRRyC3x39CDRq_3ur1=MWuUug6ov-NWmSBc3h+XyRcsgQ@mail.gmail.com> (raw)
In-Reply-To: <20200629065008.27620-5-brgl@bgdev.pl>

Hi Bartosz,

On Mon, Jun 29, 2020 at 1:56 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
>
> From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>
> Make devm_kmalloc() behave similarly to non-managed kmalloc(): return
> ZERO_SIZE_PTR when requested size is 0. Update devm_kfree() to handle
> this case.

This is wrong if you consider devm_krealloc API that you added. The
premise of devm_krealloc() is that it does not disturb devres "stack",
however in this case there is no entry in the stack. Consider:

	ptr = devm_kzalloc(dev, 0, GFP_KERNEL);
	...
	more devm API calls
	...

	/* This allocation will be on top of devm stack, not bottom ! */
	ptr = devm_krealloc(dev, ptr, 16, GFP_KERNEL);

And also:

	ptr = devm_kzalloc(dev, 16, GFP_KERNEL);
	...
	more devm API calls
	...
	/* Here we lose out position */
	ptr = devm_krealloc(dev, ptr, 0, GFP_KERNEL);
	...
	/* and now our memory allocation will be released first */
	ptr = devm_krealloc(dev, ptr, 16, GFP_KERNEL);

IMO special-casing 0-size allocations for managed memory allocations
should not be done.

Thanks.

-- 
Dmitry