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Subject: Re: [PATCH v2 2/4] mm: remove zone_lru_lock() function access
 ->lru_lock directly
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>
CC:     Johannes Weiner <hannes@cmpxchg.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Rik van Riel <riel@surriel.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>
References: <20190228083329.31892-1-aryabinin@virtuozzo.com>
 <20190228083329.31892-2-aryabinin@virtuozzo.com>
 <44ffadb4-4235-76c9-332f-680dda5da521@nvidia.com>
 <186bf66b-fec5-a614-3ffd-64b8d7660fe5@virtuozzo.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <1aac5881-0590-8c24-d963-1a7be86a7c43@nvidia.com>
Date:   Fri, 1 Mar 2019 11:58:40 -0800
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On 3/1/19 2:51 AM, Andrey Ryabinin wrote:
> 
> 
> On 3/1/19 12:44 AM, John Hubbard wrote:
>> On 2/28/19 12:33 AM, Andrey Ryabinin wrote:
>>> We have common pattern to access lru_lock from a page pointer:
>>> 	zone_lru_lock(page_zone(page))
>>>
>>> Which is silly, because it unfolds to this:
>>> 	&NODE_DATA(page_to_nid(page))->node_zones[page_zonenum(page)]->zone_pgdat->lru_lock
>>> while we can simply do
>>> 	&NODE_DATA(page_to_nid(page))->lru_lock
>>>
>>
>> Hi Andrey,
>>
>> Nice. I like it so much that I immediately want to tweak it. :)
>>
>>
>>> Remove zone_lru_lock() function, since it's only complicate things.
>>> Use 'page_pgdat(page)->lru_lock' pattern instead.
>>
>> Here, I think the zone_lru_lock() is actually a nice way to add
>> a touch of clarity at the call sites. How about, see below:
>>
>> [snip]
>>
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index 2fd4247262e9..22423763c0bd 100644
>>> --- a/include/linux/mmzone.h
>>> +++ b/include/linux/mmzone.h
>>> @@ -788,10 +788,6 @@ typedef struct pglist_data {
>>>  
>>>  #define node_start_pfn(nid)	(NODE_DATA(nid)->node_start_pfn)
>>>  #define node_end_pfn(nid) pgdat_end_pfn(NODE_DATA(nid))
>>> -static inline spinlock_t *zone_lru_lock(struct zone *zone)
>>> -{
>>> -	return &zone->zone_pgdat->lru_lock;
>>> -}
>>>  
>>
>> Instead of removing that function, let's change it, and add another
>> (since you have two cases: either a page* or a pgdat* is available),
>> and move it to where it can compile, like this:
>>
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 80bb6408fe73..cea3437f5d68 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1167,6 +1167,16 @@ static inline pg_data_t *page_pgdat(const struct page *page)
>>         return NODE_DATA(page_to_nid(page));
>>  }
>>  
>> +static inline spinlock_t *zone_lru_lock(pg_data_t *pgdat)
>> +{
>> +       return &pgdat->lru_lock;
>> +}
>> +
> 
> 
> I don't think wrapper for a simple plain access to the struct member is reasonable.
> Besides, there are plenty of "spin_lock(&pgdat->lru_lock)" even without this patch,
> so for consistency reasons &pgdat->lru_lock seems like a better choice to me.
> 
> Also "&pgdat->lru_lock" is just shorter than:
>       "node_lru_lock(pgdat)"
> 
> 
> 
>> +static inline spinlock_t *zone_lru_lock_from_page(struct page *page)
>> +{
>> +       return zone_lru_lock(page_pgdat(page));
>> +}
>> +
> 
> I don't think such function would find any use. Usually lru_lock is taken
> to perform some manipulations with page *and* pgdat, thus it's better to remember
> page_pgdat(page) in local variable.
> 

That's a good argument.

thanks,
-- 
John Hubbard
NVIDIA