2019-05-25 02:05:37

by Gen Zhang

Subject: [devm_kfree() usage] When should devm_kfree() be used?

devm_kmalloc() is used to allocate memory for a driver dev. Comments
above the definition and doc
(https://www.kernel.org/doc/Documentation/driver-model/devres.txt) all
imply that allocated the memory is automatically freed on driver attach,
no matter allocation fail or not. However, I examined the code, and
there are many sites that devm_kfree() is used to free devm_kmalloc().
e.g. hisi_sas_debugfs_init() in drivers/scsi/hisi_sas/hisi_sas_main.c.
So I am totally confused about this issue. Can anybody give me some
guidance? When should we use devm_kfree()?

Thanks
Gen

2019-06-05 06:28:32

by Jiri Slaby

Subject: Re: [devm_kfree() usage] When should devm_kfree() be used?

On 25. 05. 19, 4:03, Gen Zhang wrote:
> devm_kmalloc() is used to allocate memory for a driver dev. Comments
> above the definition and doc
> (https://www.kernel.org/doc/Documentation/driver-model/devres.txt) all
> imply that allocated the memory is automatically freed on driver attach,
> no matter allocation fail or not. However, I examined the code, and
> there are many sites that devm_kfree() is used to free devm_kmalloc().
> e.g. hisi_sas_debugfs_init() in drivers/scsi/hisi_sas/hisi_sas_main.c.

The guy who added the code in:
commit eb1c2b72b7694c984d520300c901f5fc1fa8ea9e
Author: Luo Jiaxing <[email protected]>
Date: Wed Dec 19 23:56:40 2018 +0800

scsi: hisi_sas: Alloc debugfs snapshot buffer memory for all registers

seems to be as confused as you are.

> So I am totally confused about this issue. Can anybody give me some
> guidance? When should we use devm_kfree()?

Whenever you want to free the memory earlier than on ->remove or failed
->probe. For whatever reason.

thanks,
--
js
suse labs