Re: [PATCH v2] mm: mlock: remove lru_add_drain_all()

[Shakeel Butt]

On Thu, Oct 19, 2017 at 11:19 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Thu 19-10-17 15:25:07, Shakeel Butt wrote:
>> lru_add_drain_all() is not required by mlock() and it will drain
>> everything that has been cached at the time mlock is called. And
>> that is not really related to the memory which will be faulted in
>> (and cached) and mlocked by the syscall itself.
>>
>> Without lru_add_drain_all() the mlocked pages can remain on pagevecs
>> and be moved to evictable LRUs. However they will eventually be moved
>> back to unevictable LRU by reclaim. So, we can safely remove
>> lru_add_drain_all() from mlock syscall. Also there is no need for
>> local lru_add_drain() as it will be called deep inside __mm_populate()
>> (in follow_page_pte()).
>
> This paragraph can be still a bit confusing. I suspect you meant to say
> something like: "If anything lru_add_drain_all" should be called _after_
> pages have been mlocked and faulted in but even that is not strictly
> needed because those pages would get to the appropriate LRUs lazily
> during the reclaim path. Moreover follow_page_pte (gup) will drain the
> local pcp LRU cache."
>

Andrew, can you please replace the second paragraph of the commit with
Michal's suggested paragraph.

>> On larger machines the overhead of lru_add_drain_all() in mlock() can
>> be significant when mlocking data already in memory. We have observed
>> high latency in mlock() due to lru_add_drain_all() when the users
>> were mlocking in memory tmpfs files.
>>
>> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
>
> Anyway, this patch makes a lot of sense to me. Feel free to add
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>

Thanks.