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Subject: Re: [RFC PATCH v2 1/3] meminfo_extra: introduce meminfo extra
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     leon@kernel.org, vbabka@suse.cz, adobriyan@gmail.com,
        akpm@linux-foundation.org, labbott@redhat.com,
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        kexec@lists.infradead.org
From:   Jaewon Kim <jaewon31.kim@samsung.com>
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Date:   Thu, 26 Mar 2020 17:21:28 +0900
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On 2020년 03월 24일 22:19, Greg KH wrote:
> On Tue, Mar 24, 2020 at 09:53:16PM +0900, Jaewon Kim wrote:
>>
>> On 2020년 03월 24일 20:46, Greg KH wrote:
>>> On Tue, Mar 24, 2020 at 08:37:38PM +0900, Jaewon Kim wrote:
>>>> On 2020년 03월 24일 19:11, Greg KH wrote:
>>>>> On Tue, Mar 24, 2020 at 06:11:17PM +0900, Jaewon Kim wrote:
>>>>>> On 2020년 03월 23일 18:53, Greg KH wrote:
>>>>>>>> +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 that's the case, then that's fine, it just didn't seem like that was
>>>>> needed.  Or I might have been reading your rcu logic incorrectly...
>>>>>
>>>>>>>> +	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.
>>>>> I think it is the propagation of an old and obsolete interface that you
>>>>> will have to support for the next 20+ years and yet not actually be
>>>>> useful :)
>>>>>
>>>>>> Do you think this is meaningful or cannot co-exist with other future
>>>>>> sysfs based API.
>>>>> What sysfs-based API?
>>>> Please refer to mail thread on v1 patch set - https://protect2.fireeye.com/url?k=16e3accc-4b2f6548-16e22783-0cc47aa8f5ba-935fe828ac2f6656&u=https://lkml.org/lkml/fancy/2020/3/10/2102
>>>> especially discussion with Leon Romanovsky on https://protect2.fireeye.com/url?k=74208ed9-29ec475d-74210596-0cc47aa8f5ba-0bd4ef48931fec95&u=https://lkml.org/lkml/fancy/2020/3/16/140
>>> I really do not understand what you are referring to here, sorry.   I do
>>> not see any sysfs-based code in that thread.
>> Sorry. I also did not see actual code.
>> Hello Leon Romanovsky, could you elaborate your plan regarding sysfs stuff?
>>> And try to use lore.kernel.org, lkml.org doesn't always work and we have
>>> no control over that :(
>>>
>>>>> I still don't know _why_ you want this.  The ION stuff is not needed as
>>>>> that code is about to be deleted, so who else wants this?  What is the
>>>>> use-case for it that is so desperately needed that parsing
>>>>> yet-another-proc file is going to solve the problem?
>>>> In my Android device, there are graphic driver memory, zsmalloc memory except ION.
>>> Ok, so what does Android have to do with this?
>> Some driver in Android platform may use my API to show its memory usage.
> I do not understand what this means.
>
>>>> I don't know other cases in other platform.
>>>> Not desperately needed but I think we need one userspace knob to see overall hidden huge memory.
>>> Why?  Who wants that?  What would userspace do with that?  And what
>>> exactly do you want to show?
>>>
>>> Is this just a debugging thing?  Then use debugfs for that, not proc.
>>> Isn't that what the DRM developers are starting to do?
>>>
>>>> Additionally I'd like to see all those hidden memory in OutOfMemory log.
>>> How is anything hidden, can't you see it in the slab information?
>>>
>> Let me explain more.
>>
>> 0. slab
>> As I said in cover page, this is not for memory allocated by slab.
> Great, then have the subsystem that allocates such memory, be the thing
> that exports the information.  Drivers "on their own" do not grab any
> memory without asking for it from other parts of the kernel.
>
> Modify those "other parts", this isn't a driver-specific thing at all.
>
> So, what "other parts" are involved here?
>
>> I'd like to know where huge memory has gone.
>> Those are directly allocated by alloc_pages instead of slab.
>> /proc/slabinfo does not show this information.
> Why isn't alloc_pages information exported anywhere?  Work on that.
>
>> 1. /proc/meminfo_extra
>> /proc/meminfo_extra could be debugging thing to see memory status at a certain time.
> If it is debugging, then use debugfs.
>
>> But it, I think, is also basic information rather than just for debugging.
> Who would use that information for anything except debugging?
>
>> It is similar with /proc/meminfo which is in procfs instead of debugfs.
> meminfo is older than debugfs and sysfs, can't change that today.
>
>> 2. oom log
>> oom log in show_mem is more than just debugging.
> Why?  Who sees this?
>
>> As existing oom log shows much memory information, I think we need the hidden memory info.
>> Without these information, we do NOT know oom reason because other traditional stats are not enough.
> Why not?  Kernel users of memory shouldn't be triggering OOM events.
>
>
>>>> This is useful to get clue to find memory hogger.
>>>> i.e.) show_mem on oom
>>>> <6>[  420.856428]  Mem-Info:
>>>> <6>[  420.856433]  IonSystemHeap:32813kB ZsPages:44114kB GraphicDriver::13091kB
>>>> <6>[  420.856450]  active_anon:957205 inactive_anon:159383 isolated_anon:0
>>> So what does this show you?  That someone is takign a ton of ION memory
>>> for some unknown use?  What can you do with that?  What would you do
>>> with that?
>> We may not know exact memory owner. But we can narrow down.
>> Anyway I think this is meaningful instead of no clue.
> Again, work on the subsystems that actually allocate the memory, not
> drivers.  And if you want to mess with drivers, do it in a
> device-specific way, not a driver-specific way.
>
>>> And memory is almost never assigned to a "driver", it is assigned to a
>>> "device" that uses it.  Drivers can handle multiple devices at the same
>>> time, so why would you break this down by drivers?  Are you assuming
>>> that a driver only talks to one piece of hardware?
>> Yes a driver may support several devices. I don't know if it same on an embedded device.
> Why wouldn't it be?  Is this new interface somehow only acceptable for
> systems with one-device-per-driver?  If so, that's not going to work at
> all.
>
>> Anyway I think the idea works even for several devices, although the driver should
>> distinguish memory usage for each device and should register each memory stat.
> And how would that happen?
>
> thanks,
>
> greg k-h
>
>
Thank you for you guys' comment
Let me consider more