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Date: Tue, 12 Dec 2017 14:45:07 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: weiping zhang <zwp10758@gmail.com>
Cc: linux-kernel@vger.kernel.org, Cornelia Huck <cohuck@redhat.com>,
        weiping zhang <zhangweiping@didichuxing.com>,
        virtualization@lists.linux-foundation.org,
        "Michael S . Tsirkin" <mst@redhat.com>
Subject: Re: [PATCHv2] virtio_mmio: fix devm cleanup
Message-ID: <20171212144506.c4z6aqieo5uo3pcn@lakrids.cambridge.arm.com>
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On Tue, Dec 12, 2017 at 10:26:24PM +0800, weiping zhang wrote:
> 2017-12-12 21:45 GMT+08:00 Mark Rutland <mark.rutland@arm.com>:
> Hi Mark,

Hi,

> thanks your patch, I dig into these three devm_xxx funciton,
> all of them represented by a struct devres as following,
> 
> struct devres_node {
>         struct list_head                entry;
>         dr_release_t                    release;
> #ifdef CONFIG_DEBUG_DEVRES
>         const char                      *name;
>         size_t                          size;
> #endif
> 
> };
> 
> struct devres {
>         struct devres_node              node;
>         /* -- 3 pointers */
>         unsigned long long              data[]; /* guarantee ull alignment */
> };

> 2) devm_kzalloc -> devm_kmalloc
> 
> dr = alloc_dr(devm_kmalloc_release, size, gfp, dev_to_node(dev));
> "devm_kmalloc_release" is noop, do nothing.

Please note that the release function is there to perform cleanup prior
to the devm infrastructure releasing the memory.

The devm_kmalloc_release function is a no-op since nothing has to be
done prior to memory being freed, but the memory itself is still freed.

In alloc_dr(), the struct devres is allocated together with the memory,
since alloc_dr() does:

	size_t tot_size = sizeof(struct devres) + size; 
	struct devres *dr;

	dr = kmalloc_node_track_caller(tot_size, gfp, nid);

	return dr->data;

... where dr->data points at the memory after the struct devres.

Later, in release_nodes() we do:

	list_for_each_entry_safe_reverse(dr, tmp, &todo, node.entry) {
		devres_log(dev, &dr->node, "REL");
		dr->node.release(dev, dr->data);
		kfree(dr);
	}    

... which will invoke the no-op devm_kmalloc_release, then free the
devres allocation, including the dr->data memory the user requested.

> so for case 2) above, we need a devm_kfree() before call
> register_virtio_device

As above, I do not believe that is the case.

Thanks,
Mark.