Re: [PATCH] mm, show_mem: drop pgdat_resize_lock in show_mem()

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, Nov 29, 2018 at 09:17:03AM +0100, Michal Hocko wrote:
>On Thu 29-11-18 05:08:15, Wei Yang wrote:
>> Function show_mem() is used to print system memory status when user
>> requires or fail to allocate memory. Generally, this is a best effort
>> information and not willing to affect core mm subsystem.
>
>I would drop the part after and
>
>> The data protected by pgdat_resize_lock is mostly correct except there is:
>> 
>>    * page struct defer init
>>    * memory hotplug
>
>This is more confusing than helpful. I would just drop it.
>
>The changelog doesn't explain what is done and why. The second one is
>much more important. I would say this
>
>"
>Function show_mem() is used to print system memory status when user
>requires or fail to allocate memory. Generally, this is a best effort
>information so any races with memory hotplug (or very theoretically an
>early initialization) should be toleratable and the worst that could
>happen is to print an imprecise node state.
>
>Drop the resize lock because this is the only place which might hold the
>lock from the interrupt context and so all other callers might use a
>simple spinlock. Even though this doesn't solve any real issue it makes
>the code easier to follow and tiny more effective.
>"

Ah, I have to admit this is much clearer and easier for audience to
understand the reason.

Thanks a lot.

>
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> ---
>>  lib/show_mem.c | 2 --
>>  1 file changed, 2 deletions(-)
>> 
>> diff --git a/lib/show_mem.c b/lib/show_mem.c
>> index 0beaa1d899aa..1d996e5771ab 100644
>> --- a/lib/show_mem.c
>> +++ b/lib/show_mem.c
>> @@ -21,7 +21,6 @@ void show_mem(unsigned int filter, nodemask_t *nodemask)
>>  		unsigned long flags;
>
>btw. you want to drop flags.

Oops, what a shame . :-(

>>  		int zoneid;
>>  
>> -		pgdat_resize_lock(pgdat, &flags);
>>  		for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
>>  			struct zone *zone = &pgdat->node_zones[zoneid];
>>  			if (!populated_zone(zone))
>> @@ -33,7 +32,6 @@ void show_mem(unsigned int filter, nodemask_t *nodemask)
>>  			if (is_highmem_idx(zoneid))
>>  				highmem += zone->present_pages;
>>  		}
>> -		pgdat_resize_unlock(pgdat, &flags);
>>  	}
>>  
>>  	printk("%lu pages RAM\n", total);
>> -- 
>> 2.15.1
>> 
>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me