linux-kernel - Re: [PATCH v6] mm: cma: support sysfs

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Message-ID: <71b7914d-d9ff-2200-d6c9-6eab28499887@nvidia.com>
Date:   Tue, 23 Mar 2021 23:26:08 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        <gregkh@...uxfoundation.org>, <surenb@...gle.com>,
        <joaodias@...gle.com>, <willy@...radead.org>, <digetx@...il.com>
Subject: Re: [PATCH v6] mm: cma: support sysfs

On 3/23/21 10:44 PM, Minchan Kim wrote:
> On Tue, Mar 23, 2021 at 09:47:27PM -0700, John Hubbard wrote:
>> On 3/23/21 8:27 PM, Minchan Kim wrote:
>> ...
>>>>> +static int __init cma_sysfs_init(void)
>>>>> +{
>>>>> +	unsigned int i;
>>>>> +
>>>>> +	cma_kobj_root = kobject_create_and_add("cma", mm_kobj);
>>>>> +	if (!cma_kobj_root)
>>>>> +		return -ENOMEM;
>>>>> +
>>>>> +	for (i = 0; i < cma_area_count; i++) {
>>>>> +		int err;
>>>>> +		struct cma *cma;
>>>>> +		struct cma_kobject *cma_kobj;
>>>>> +
>>>>> +		cma_kobj = kzalloc(sizeof(*cma_kobj), GFP_KERNEL);
>>>>> +		if (!cma_kobj) {
>>>>> +			kobject_put(cma_kobj_root);
>>>>> +			return -ENOMEM;
>>>>
>>>> This leaks little cma_kobj's all over the floor. :)
>>>
>>> I thought kobject_put(cma_kobj_root) should deal with it. No?
>>>
>> If this fails when i > 0, there will be cma_kobj instances that
>> were stashed in the cma_areas[] array. But this code only deletes
>> the most recently allocated cma_kobj, not anything allocated on
>> previous iterations of the loop.
> 
> Oh, I misunderstood that destroying of root kobject will release
> children recursively. Seems not true. Go back to old version.
> 
> 
> index 16c81c9cb9b7..418951a3f138 100644
> --- a/mm/cma_sysfs.c
> +++ b/mm/cma_sysfs.c
> @@ -80,20 +80,19 @@ static struct kobj_type cma_ktype = {
>   static int __init cma_sysfs_init(void)
>   {
>          unsigned int i;
> +       int err;
> +       struct cma *cma;
> +       struct cma_kobject *cma_kobj;
> 
>          cma_kobj_root = kobject_create_and_add("cma", mm_kobj);
>          if (!cma_kobj_root)
>                  return -ENOMEM;
> 
>          for (i = 0; i < cma_area_count; i++) {
> -               int err;
> -               struct cma *cma;
> -               struct cma_kobject *cma_kobj;
> -
>                  cma_kobj = kzalloc(sizeof(*cma_kobj), GFP_KERNEL);
>                  if (!cma_kobj) {
> -                       kobject_put(cma_kobj_root);
> -                       return -ENOMEM;
> +                       err = -ENOMEM;
> +                       goto out;
>                  }
> 
>                  cma = &cma_areas[i];
> @@ -103,11 +102,21 @@ static int __init cma_sysfs_init(void)
>                                             cma_kobj_root, "%s", cma->name);
>                  if (err) {
>                          kobject_put(&cma_kobj->kobj);
> -                       kobject_put(cma_kobj_root);
> -                       return err;
> +                       goto out;
>                  }
>          }
> 
>          return 0;
> +out:
> +       while (--i >= 0) {
> +               cma = &cma_areas[i];
> +
> +               kobject_put(&cma->kobj->kobj);


OK. As long as you are spinning a new version, let's fix up the naming to
be a little better, too. In this case, with a mildly dizzying mix of cma's
and kobjects, it actually makes a real difference. I wouldn't have asked,
but the above cma->kobj->kobj chain really made it obvious for me just now.

So instead of this (in cma.h):

struct cma_kobject {
	struct cma *cma;
	struct kobject kobj;
};

struct cma {
...
	struct cma_kobject *kobj;
};

, how about approximately this:

struct cma_kobject_wrapper {
	struct cma *parent;
	struct kobject kobj;
};

struct cma {
...
	struct cma_kobject_wrapper *cma_kobj_wrapper;
};


...thus allowing readers of cma_sysfs.c to read that file more easily.


> +               kfree(cma->kobj);
> +               cma->kobj = NULL;
> +       }
> +       kobject_put(cma_kobj_root);
> +
> +       return err;
>   }
>   subsys_initcall(cma_sysfs_init);
> 
> 
> 

thanks,
-- 
John Hubbard
NVIDIA