linux-kernel - Re: [PATCH -V4 -mm] mm, swap: Fix race between swapoff and some swap operations

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Message-ID: <20180104102114.l45sjluuzdgpcfd7@techsingularity.net>
Date:   Thu, 4 Jan 2018 10:21:14 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Jan Kara <jack@...e.cz>, Mel Gorman <mgorman@...e.de>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Shaohua Li <shli@...com>,
        J???r???me Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Aaron Lu <aaron.lu@...el.com>
Subject: Re: [PATCH -V4 -mm] mm, swap: Fix race between swapoff and some swap
 operations

On Thu, Jan 04, 2018 at 09:17:36AM +0800, Huang, Ying wrote:
> > Maybe, but in this particular case, I would prefer to go with something
> > more conventional unless there is strong evidence that it's an improvement
> > (which I doubt in this case given the cost of migration overall and the
> > corner case of migrating a dirty page).
> 
> So you like page_lock() more than RCU? 

In this instance, yes.

> Is there any problem of RCU?
> The object to be protected isn't clear?
> 

It's not clear what object is being protected or how it's protected and
it's not the usual means a mapping is pinned. Furthermore, in the event
a page is being truncated, we really do not want to bother doing any
migration work for compaction purposes as it's a waste.

-- 
Mel Gorman
SUSE Labs