linux-kernel - Re: [PATCH v2] mm: mlock: remove lru_add_drain

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Date:   Fri, 20 Oct 2017 08:07:19 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Minchan Kim <minchan@...nel.org>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        Ingo Molnar <mingo@...nel.org>,
        Greg Thelen <gthelen@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Balbir Singh <bsingharora@...il.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: mlock: remove lru_add_drain_all()

On Thu, Oct 19, 2017 at 11:19 PM, Michal Hocko <mhocko@...nel.org> 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@...gle.com>
>
> Anyway, this patch makes a lot of sense to me. Feel free to add
> Acked-by: Michal Hocko <mhocko@...e.com>
>

Thanks.