linux-kernel - Re: [RFC PATCH v2 1/2] kernfs: use kernfs

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Date:   Tue, 4 Jan 2022 16:48:35 +1100
From:   Imran Khan <imran.f.khan@...cle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 1/2] kernfs: use kernfs_node specific mutex and
 spinlock.

Hi Greg

On 4/1/22 9:16 am, Imran Khan wrote:
> 
> 
> On 3/1/22 8:54 pm, Greg KH wrote:
>> On Mon, Jan 03, 2022 at 07:45:43PM +1100, Imran Khan wrote:
>>> diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
>>> index 861c4f0f8a29..5ed4c9ed39af 100644
>>> --- a/include/linux/kernfs.h
>>> +++ b/include/linux/kernfs.h
>>> @@ -164,6 +164,8 @@ struct kernfs_node {
>>>  	unsigned short		flags;
>>>  	umode_t			mode;
>>>  	struct kernfs_iattrs	*iattr;
>>> +	spinlock_t kernfs_open_node_lock;
>>> +	struct mutex kernfs_open_file_mutex;
>>
>> Did you just blow up the memory requirements of a system with lots of
>> kobjects created?
>>> We used to be able to support tens of thousands of scsi devices in a
>> 32bit kernel, with this change, what is the memory difference that just
>> happened?
>>
> Indeed, this patch increases kernfs_node size by 36 bytes ( 28 bytes for
> mutex + 4 bytes for spinlock). From current kernfs_node size of 128
> bytes, this will be a ~25% increase in kobjects memory consumption.
> I can replace the mutex object with a pointer to it, to bring down
> the overall increase in size. Will the size change be acceptable then?
> 

Please ignore my proposal about using pointers to mutex object. It will
reduce the size of kernfs_node object but eventually overall kobject
memory usage will not reduce.

Thanks,
Imran