linux-kernel - Re: [PATCH] mm, mlock, vmscan: no more skipping pagevecs

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Date:   Tue, 21 Nov 2017 09:20:16 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Huang Ying <ying.huang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Balbir Singh <bsingharora@...il.com>,
        Minchan Kim <minchan@...nel.org>, Shaohua Li <shli@...com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Nicholas Piggin <npiggin@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, mlock, vmscan: no more skipping pagevecs

On Tue, Nov 21, 2017 at 7:32 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> On Sat, Nov 04, 2017 at 03:43:12PM -0700, Shakeel Butt wrote:
>> When a thread mlocks an address space backed by file, a new
>> page is allocated (assuming file page is not in memory), added
>> to the local pagevec (lru_add_pvec), I/O is triggered and the
>> thread then sleeps on the page. On I/O completion, the thread
>> can wake on a different CPU, the mlock syscall will then sets
>> the PageMlocked() bit of the page but will not be able to put
>> that page in unevictable LRU as the page is on the pagevec of
>> a different CPU. Even on drain, that page will go to evictable
>> LRU because the PageMlocked() bit is not checked on pagevec
>> drain.
>>
>> The page will eventually go to right LRU on reclaim but the
>> LRU stats will remain skewed for a long time.
>>
>> However, this issue does not happen for anon pages on swap
>> because unlike file pages, anon pages are not added to pagevec
>> until they have been fully swapped in.
>
> How so? __read_swap_cache_async() is the core function that allocates
> the page, and that always puts the page on the pagevec before IO is
> initiated.
>
>> Also the fault handler uses vm_flags to set the PageMlocked() bit of
>> such anon pages even before returning to mlock() syscall and mlocked
>> pages will skip pagevecs and directly be put into unevictable LRU.
>
> Where does the swap fault path set PageMlocked()?
>
> I might just be missing something.

No, you are right. I got confused by
lru_cache_add_active_or_unevictable() in do_swap_page() but missed the
preceding comment that says "ksm created a completely new copy". I
will fix the the commit message as well.