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

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Date:   Tue, 21 Nov 2017 10:32:57 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shakeel Butt <shakeelb@...gle.com>
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@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, mlock, vmscan: no more skipping pagevecs

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.