linux-kernel - Re: [PATCH v3 2/4] powerpc/fadump: reorganize /sys/kernel/fadump

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Date:   Tue, 3 Dec 2019 16:50:32 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
To:     Michal Suchánek <msuchanek@...e.de>
Cc:     corbet@....net, mahesh@...ux.vnet.ibm.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...abs.org, hbathini@...ux.ibm.com
Subject: Re: [PATCH v3 2/4] powerpc/fadump: reorganize /sys/kernel/fadump_*
 sysfs files

>>>>  DEFINE_SHOW_ATTRIBUTE(fadump_region);
>>>>  
>>>>  static void fadump_init_files(void)
>>>> @@ -1435,6 +1448,11 @@ static void fadump_init_files(void)
>>>>  	struct dentry *debugfs_file;
>>>>  	int rc = 0;
>>>>  
>>>> +	fadump_kobj = kobject_create_and_add("fadump", kernel_kobj);
>>>> +	if (!fadump_kobj) {
>>>> +		pr_err("failed to create fadump kobject\n");
>>>> +		return;
>>>> +	}
>>>>  	rc = sysfs_create_file(kernel_kobj, &fadump_attr.attr);
>>>>  	if (rc)
>>>>  		printk(KERN_ERR "fadump: unable to create sysfs file"
>>>> @@ -1458,6 +1476,26 @@ static void fadump_init_files(void)
>>>>  			printk(KERN_ERR "fadump: unable to create sysfs file"
>>>>  				" fadump_release_mem (%d)\n", rc);
>>>>  	}
>>>> +	/* Replicating the following sysfs attributes under FADump kobject.
>>>> +	 *
>>>> +	 *	- fadump_enabled -> enabled
>>>> +	 *	- fadump_registered -> registered
>>>> +	 *	- fadump_release_mem -> release_mem
>>>> +	 */
>>>> +	rc = sysfs_create_file(fadump_kobj, &enable_attr.attr);
>>>> +	if (rc)
>>>> +		pr_err("unable to create enabled sysfs file (%d)\n",
>>>> +		       rc);
>>>> +	rc = sysfs_create_file(fadump_kobj, &register_attr.attr);
>>>> +	if (rc)
>>>> +		pr_err("unable to create registered sysfs file (%d)\n",
>>>> +		       rc);
>>>> +	if (fw_dump.dump_active) {
>>>> +		rc = sysfs_create_file(fadump_kobj, &release_attr.attr);
>>>> +		if (rc)
>>>> +			pr_err("unable to create release_mem sysfs file (%d)\n",
>>>> +			       rc);
>>>> +	}
>>>>  	return;
>>>>  }
>>> Hello,
>>>
>>
>> I’m so sorry for taking this long to write you back. 
>>
>>> wouldn't it make more sense to create the files in the new location and
>>> add a symlink at the old location?
>>
>> There are APIs which allow to create a symlink for an entire kobject but
>> I did not find a way to create symlink of an individual sysfs file.
>>
>> Do you have any approach in mind to achieve the same?
> 
> There is at least one example of plain symlink:
> 
> find /sys -type l -xtype f
> /sys/kernel/security/evm
> 
> If there is no interface to do one sanely duplicationg the files is
> better than nothing.

Hello Michal,

I found a function (__compat_only_sysfs_link_entry_to_kobj) that adds a symlink
to a kobject.

But the problem is __compat_only_sysfs_link_entry_to_kobj function keeps the
symlink file name similar to sysfs file and has no option to change it.

We can't use the __compat_only_sysfs_link_entry_to_kobj function directly because
our symlink file name must be different from the target file name.

	fadump_enabled -> fadump/enabled

But the good thing is we can tweak the __compat_only_sysfs_link_entry_to_kobj
function and allow the caller to change the sysmlink file name.

So I am writing a separate patch that adds a wrapper function around the __compat_only_sysfs_link_entry_to_kobj function which will allow to have a custom symlink file name.

Thanks,
Sourabh Jain