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

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Date:   Fri, 20 Dec 2019 15:16:05 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        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 03:06:55PM +0100, Peter Zijlstra wrote:
> 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);
> 	}
> }
> 

Ok, that's my queue to walk away from the keyboard and start drinking, I
think the holiday season has now officially started.

ugh.  crazy.

but brilliant :)

greg k-h