linux-kernel - Re: [RFC PATCH v1] devres: align devres.data strictly only for devm

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Date:   Fri, 20 Dec 2019 15:06:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marc Gonzalez <marc.w.gonzalez@...e.fr>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Robin Murphy <robin.murphy@....com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        Will Deacon <will@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Tejun Heo <tj@...nel.org>, Mark Brown <broonie@...nel.org>
Subject: Re: [RFC PATCH v1] devres: align devres.data strictly only for
 devm_kmalloc()

On Fri, Dec 20, 2019 at 11:19:27AM +0100, Marc Gonzalez wrote:
> Would anyone else have any suggestions, comments, insights, recommendations,
> improvements, guidance, or wisdom? :-)

Flip devres upside down!

**WARNING, wear protective glasses when reading the below**


struct devres {
	struct devres_node	node;
	void			*data;
};

/*
 * We place struct devres at the tail of the memory allocation
 * such that data retains the ARCH_KMALLOC_MINALIGN alignment.
 * struct devres itself is just 4 pointers and should therefore
 * only require trivial alignment.
 */
static inline struct devres *data2devres(void *data)
{
	return (struct devres *)(data + ksize(data) - sizeof(struct devres));
}

void *alloc_dr(...)
{
	struct devres *dr;
	void *data;

	data = kmalloc(size + sizeof(struct devres), GFP_KERNEL);
	dr = data2devres(data);
	WARN_ON((unsigned long)dr & __alignof(*dr)-1);
	INIT_LIST_HEAD(&dr->node.entry);
	dr->node.release = release;
	dr->data = data;

	return dr;
}

void devres_free(void *data)
{
	if (data) {
		struct devres *dr = data2devres(data);
		BUG_ON(!list_empty(dr->node.entry));
		kfree(data);
	}
}

static int release_nodes(...)
{
	...
	list_for_each_entry_safe_reverse(dr, ...) {
		...
		kfree(dr->data);
	}
}