Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v3 2/4] powerpc/fadump: reorganize /sys/kernel/fadump_*
 sysfs files
To:     =?UTF-8?Q?Michal_Such=c3=a1nek?= <msuchanek@suse.de>
Cc:     corbet@lwn.net, mahesh@linux.vnet.ibm.com,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linuxppc-dev@ozlabs.org, hbathini@linux.ibm.com
References: <20191109122339.20484-1-sourabhjain@linux.ibm.com>
 <20191109122339.20484-3-sourabhjain@linux.ibm.com>
 <20191109125933.GF1384@kitsune.suse.cz>
 <8c1ec297-0c34-12de-2528-be436697215a@linux.ibm.com>
 <20191124184059.GP11661@kitsune.suse.cz>
From:   Sourabh Jain <sourabhjain@linux.ibm.com>
Date:   Wed, 27 Nov 2019 10:24:06 +0530
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.4.0
MIME-Version: 1.0
In-Reply-To: <20191124184059.GP11661@kitsune.suse.cz>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Message-Id: <ea97b5e3-e1e2-2aff-8227-7cfb551c8ebc@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 11/25/19 12:10 AM, Michal Suchánek wrote:
> On Sat, Nov 16, 2019 at 08:07:29PM +0530, Sourabh Jain wrote:
>>
>>
>> On 11/9/19 6:29 PM, Michal Suchánek wrote:
>>> On Sat, Nov 09, 2019 at 05:53:37PM +0530, Sourabh Jain wrote:
>>>> As the number of FADump sysfs files increases it is hard to manage all of
>>>> them inside /sys/kernel directory. It's better to have all the FADump
>>>> related sysfs files in a dedicated directory /sys/kernel/fadump. But in
>>>> order to maintain the backward compatibility the /sys/kernel/fadump_*
>>>> sysfs files are replicated inside /sys/kernel/fadump/ and eventually get
>>>> removed in future.
>>>>
>>>> As the FADump sysfs files are now part of dedicated directory there is no
>>>> need to prefix their name with fadump_, hence sysfs file names are also
>>>> updated. For example fadump_enabled sysfs file is now referred as enabled.
>>>>
>>>> Also consolidate ABI documentation for all the FADump sysfs files in a
>>>> single file Documentation/ABI/testing/sysfs-kernel-fadump.
>>>>
>>>> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
>>>> ---
>>>>  Documentation/ABI/testing/sysfs-kernel-fadump | 41 +++++++++++++++++++
>>>>  arch/powerpc/kernel/fadump.c                  | 38 +++++++++++++++++
>>>>  arch/powerpc/platforms/powernv/opal-core.c    | 10 +++--
>>>>  3 files changed, 86 insertions(+), 3 deletions(-)
>>>>  create mode 100644 Documentation/ABI/testing/sysfs-kernel-fadump
>>>>
>>>> diff --git a/Documentation/ABI/testing/sysfs-kernel-fadump b/Documentation/ABI/testing/sysfs-kernel-fadump
>>>> new file mode 100644
>>>> index 000000000000..a77f1a5ba389
>>>> --- /dev/null
>>>> +++ b/Documentation/ABI/testing/sysfs-kernel-fadump
>>>> @@ -0,0 +1,41 @@
>>>> +What:		/sys/kernel/fadump/*
>>>> +Date:		Nov 2019
>>>> +Contact:	linuxppc-dev@lists.ozlabs.org
>>>> +Description:
>>>> +		The /sys/kernel/fadump/* is a collection of FADump sysfs
>>>> +		file provide information about the configuration status
>>>> +		of Firmware Assisted Dump (FADump).
>>>> +
>>>> +What:		/sys/kernel/fadump/enabled
>>>> +Date:		Nov 2019
>>>> +Contact:	linuxppc-dev@lists.ozlabs.org
>>>> +Description:	read only
>>>> +		Primarily used to identify whether the FADump is enabled in
>>>> +		the kernel or not.
>>>> +User:		Kdump service
>>>> +
>>>> +What:		/sys/kernel/fadump/registered
>>>> +Date:		Nov 2019
>>>> +Contact:	linuxppc-dev@lists.ozlabs.org
>>>> +Description:	read/write
>>>> +		Helps to control the dump collect feature from userspace.
>>>> +		Setting 1 to this file enables the system to collect the
>>>> +		dump and 0 to disable it.
>>>> +User:		Kdump service
>>>> +
>>>> +What:		/sys/kernel/fadump/release_mem
>>>> +Date:		Nov 2019
>>>> +Contact:	linuxppc-dev@lists.ozlabs.org
>>>> +Description:	write only
>>>> +		This is a special sysfs file and only available when
>>>> +		the system is booted to capture the vmcore using FADump.
>>>> +		It is used to release the memory reserved by FADump to
>>>> +		save the crash dump.
>>>> +
>>>> +What:		/sys/kernel/fadump/release_opalcore
>>>> +Date:		Nov 2019
>>>> +Contact:	linuxppc-dev@lists.ozlabs.org
>>>> +Description:	write only
>>>> +		The sysfs file is available when the system is booted to
>>>> +		collect the dump on OPAL based machine. It used to release
>>>> +		the memory used to collect the opalcore.
>>>> diff --git a/arch/powerpc/kernel/fadump.c b/arch/powerpc/kernel/fadump.c
>>>> index ed59855430b9..a9591def0c84 100644
>>>> --- a/arch/powerpc/kernel/fadump.c
>>>> +++ b/arch/powerpc/kernel/fadump.c
>>>> @@ -1418,6 +1418,9 @@ static int fadump_region_show(struct seq_file *m, void *private)
>>>>  	return 0;
>>>>  }
>>>>  
>>>> +struct kobject *fadump_kobj;
>>>> +EXPORT_SYMBOL_GPL(fadump_kobj);
>>>> +
>>>>  static struct kobj_attribute fadump_release_attr = __ATTR(fadump_release_mem,
>>>>  						0200, NULL,
>>>>  						fadump_release_memory_store);
>>>> @@ -1428,6 +1431,16 @@ static struct kobj_attribute fadump_register_attr = __ATTR(fadump_registered,
>>>>  						0644, fadump_register_show,
>>>>  						fadump_register_store);
>>>>  
>>>> +static struct kobj_attribute release_attr = __ATTR(release_mem,
>>>> +						0200, NULL,
>>>> +						fadump_release_memory_store);
>>>> +static struct kobj_attribute enable_attr = __ATTR(enabled,
>>>> +						0444, fadump_enabled_show,
>>>> +						NULL);
>>>> +static struct kobj_attribute register_attr = __ATTR(registered,
>>>> +						0644, fadump_register_show,
>>>> +						fadump_register_store);
>>>> +
>>>>  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

Yes, there are APIs available in debugfs and securityfs that allow creatinga symlink of sysfs files. But I did not find a generic API at sysfs level tocreate symlink.

Let's wait for others to put in their view on this. Meanwhile, I'll start
exploring how we can replace the older FADump sysfs files with symlink.

Thanks,
Sourabh Jain