Re: [RFC PATCH v2 1/3] meminfo_extra: introduce meminfo extra

[Jaewon Kim]

On 2020ë 03ì 23ì 18:53, Greg KH wrote:
> On Mon, Mar 23, 2020 at 05:05:01PM +0900, Jaewon Kim wrote:
>> Provide APIs to drivers so that they can show its memory usage on
>> /proc/meminfo_extra.
>>
>> int register_meminfo_extra(atomic_long_t *val, int shift,
>> 			   const char *name);
>> int unregister_meminfo_extra(atomic_long_t *val);
> Nit, isn't it nicer to have the subsystem name first:
> 	meminfo_extra_register()
> 	meminfo_extra_unregister()
> ?
OK. Name can be changed.
>
>
>> Signed-off-by: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
>> ---
>> v2: move to /proc/meminfo_extra as a new file, meminfo_extra.c
>>     use rcu to reduce lock overhead
>> v1: print info at /proc/meminfo
>> ---
>>  fs/proc/Makefile        |   1 +
>>  fs/proc/meminfo_extra.c | 123 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  include/linux/mm.h      |   4 ++
>>  mm/page_alloc.c         |   1 +
>>  4 files changed, 129 insertions(+)
>>  create mode 100644 fs/proc/meminfo_extra.c
>>
>> diff --git a/fs/proc/Makefile b/fs/proc/Makefile
>> index bd08616ed8ba..83d2f55591c6 100644
>> --- a/fs/proc/Makefile
>> +++ b/fs/proc/Makefile
>> @@ -19,6 +19,7 @@ proc-y	+= devices.o
>>  proc-y	+= interrupts.o
>>  proc-y	+= loadavg.o
>>  proc-y	+= meminfo.o
>> +proc-y	+= meminfo_extra.o
>>  proc-y	+= stat.o
>>  proc-y	+= uptime.o
>>  proc-y	+= util.o
>> diff --git a/fs/proc/meminfo_extra.c b/fs/proc/meminfo_extra.c
>> new file mode 100644
>> index 000000000000..bd3f0d2b7fb7
>> --- /dev/null
>> +++ b/fs/proc/meminfo_extra.c
>> @@ -0,0 +1,123 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +#include <linux/mm.h>
>> +#include <linux/proc_fs.h>
>> +#include <linux/seq_file.h>
>> +#include <linux/slab.h>
>> +
>> +static void show_val_kb(struct seq_file *m, const char *s, unsigned long num)
>> +{
>> +	seq_put_decimal_ull_width(m, s, num << (PAGE_SHIFT - 10), 8);
>> +	seq_write(m, " kB\n", 4);
>> +}
>> +
>> +static LIST_HEAD(meminfo_head);
>> +static DEFINE_SPINLOCK(meminfo_lock);
>> +
>> +#define NAME_SIZE      15
>> +#define NAME_BUF_SIZE  (NAME_SIZE + 2) /* ':' and '\0' */
>> +
>> +struct meminfo_extra {
>> +	struct list_head list;
>> +	atomic_long_t *val;
>> +	int shift_for_page;
>> +	char name[NAME_BUF_SIZE];
>> +	char name_pad[NAME_BUF_SIZE];
>> +};
>> +
>> +int register_meminfo_extra(atomic_long_t *val, int shift, const char *name)
>> +{
>> +	struct meminfo_extra *meminfo, *memtemp;
>> +	int len;
>> +	int error = 0;
>> +
>> +	meminfo = kzalloc(sizeof(*meminfo), GFP_KERNEL);
>> +	if (!meminfo) {
>> +		error = -ENOMEM;
>> +		goto out;
>> +	}
>> +
>> +	meminfo->val = val;
>> +	meminfo->shift_for_page = shift;
>> +	strncpy(meminfo->name, name, NAME_SIZE);
>> +	len = strlen(meminfo->name);
>> +	meminfo->name[len] = ':';
>> +	strncpy(meminfo->name_pad, meminfo->name, NAME_BUF_SIZE);
>> +	while (++len < NAME_BUF_SIZE - 1)
>> +		meminfo->name_pad[len] = ' ';
>> +
>> +	spin_lock(&meminfo_lock);
>> +	list_for_each_entry_rcu(memtemp, &meminfo_head, list) {
>> +		if (memtemp->val == val) {
>> +			error = -EINVAL;
>> +			break;
>> +		}
>> +	}
>> +	if (!error)
>> +		list_add_tail_rcu(&meminfo->list, &meminfo_head);
>> +	spin_unlock(&meminfo_lock);
> If you have a lock, why are you needing rcu?
I think _rcu should be removed out of list_for_each_entry_rcu.
But I'm confused about what you meant.
I used rcu_read_lock on __meminfo_extra,
and I think spin_lock is also needed for addition and deletion to handle multiple modifiers.
>
>
>
>> +	if (error)
>> +		kfree(meminfo);
>> +out:
>> +
>> +	return error;
>> +}
>> +EXPORT_SYMBOL(register_meminfo_extra);
> EXPORT_SYMBOL_GPL()?  I have to ask :)
I can use EXPORT_SYMBOL_GPL.
>
> thanks,
>
> greg k-h
>
>

Hello
Thank you for your comment.

By the way there was not resolved discussion on v1 patch as I mentioned on cover page.
I'd like to hear your opinion on this /proc/meminfo_extra node.
Do you think this is meaningful or cannot co-exist with other future sysfs based API.