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SubjectRe: [PATCHv2] virtio_mmio: fix devm cleanup
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.

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