linux-kernel - Re: [PATCH v2 4/6] devres: handle zero size in devm

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Message-ID: <CAKdAkRRRyC3x39CDRq_3ur1=MWuUug6ov-NWmSBc3h+XyRcsgQ@mail.gmail.com>
Date:   Sat, 10 Apr 2021 20:21:24 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jean Delvare <jdelvare@...e.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, linux-hwmon@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v2 4/6] devres: handle zero size in devm_kmalloc()

Hi Bartosz,

On Mon, Jun 29, 2020 at 1:56 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
>
> From: Bartosz Golaszewski <bgolaszewski@...libre.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