Re: [PATCH v7 02/10] mm/memcg: fold lru_lock in lock_page_lru

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 13/01/2020 12.45, Alex Shi wrote:
> 
> 
> 在 2020/1/10 下午4:49, Konstantin Khlebnikov 写道:
>> On 25/12/2019 12.04, Alex Shi wrote:
>>>   From the commit_charge's explanations and mem_cgroup_commit_charge
>>> comments, as well as call path when lrucare is ture, The lru_lock is
>>> just to guard the task migration(which would be lead to move_account)
>>> So it isn't needed when !PageLRU, and better be fold into PageLRU to
>>> reduce lock contentions.
>>>
>>> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>> Cc: Michal Hocko <mhocko@kernel.org>
>>> Cc: Matthew Wilcox <willy@infradead.org>
>>> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: cgroups@vger.kernel.org
>>> Cc: linux-mm@kvack.org
>>> Cc: linux-kernel@vger.kernel.org
>>> ---
>>>    mm/memcontrol.c | 9 ++++-----
>>>    1 file changed, 4 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index c5b5f74cfd4d..0ad10caabc3d 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -2572,12 +2572,11 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
>>>      static void lock_page_lru(struct page *page, int *isolated)
>>>    {
>>> -    pg_data_t *pgdat = page_pgdat(page);
>>> -
>>> -    spin_lock_irq(&pgdat->lru_lock);
>>>        if (PageLRU(page)) {
>>> +        pg_data_t *pgdat = page_pgdat(page);
>>>            struct lruvec *lruvec;
>>>    +        spin_lock_irq(&pgdat->lru_lock);
>>
>> That's wrong. Here PageLRU must be checked again under lru_lock.
> Hi, Konstantin,
> 
> For logical remain, we can get the lock and then release for !PageLRU.
> but I still can figure out the problem scenario. Would like to give more hints?

That's trivial race: page could be isolated from lru between

if (PageLRU(page))
and
spin_lock_irq(&pgdat->lru_lock);

> 
> 
>>
>>
>> Also I don't like these functions:
>> - called lock/unlock but actually also isolates
>> - used just once
>> - pgdat evaluated twice
> 
> That's right. I will fold these functions into commit_charge.
> 
> Thanks
> Alex
>