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Date: Tue, 21 Nov 2017 10:32:57 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Shakeel Butt <shakeelb@google.com>
Cc: Huang Ying <ying.huang@intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
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        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
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        Hugh Dickins <hughd@google.com>, Vlastimil Babka <vbabka@suse.cz>,
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Subject: Re: [PATCH] mm, mlock, vmscan: no more skipping pagevecs
Message-ID: <20171121153257.GA23920@cmpxchg.org>
References: <20171104224312.145616-1-shakeelb@google.com>
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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.