linux-kernel - Re: [PATCH 2/8] mm/swap: fix race on swap_info reuse between swapoff and swapon

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Date:	Mon, 3 Feb 2014 20:14:01 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Weijie Yang <weijie.yang@...sung.com>, hughd@...gle.com,
	Minchan Kim <minchan@...nel.org>, shli@...nel.org,
	Bob Liu <bob.liu@...cle.com>, weijie.yang.kh@...il.com,
	Seth Jennings <sjennings@...iantweb.net>,
	Heesub Shin <heesub.shin@...sung.com>, mquzik@...hat.com,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/8] mm/swap: fix race on swap_info reuse between swapoff
 and swapon

On Mon, 3 Feb 2014, Andrew Morton wrote:
> On Mon, 3 Feb 2014 15:23:40 -0800 Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Mon, 27 Jan 2014 18:03:04 +0800 Weijie Yang <weijie.yang@...sung.com> wrote:
> > 
> > > swapoff clear swap_info's SWP_USED flag prematurely and free its resources
> > > after that. A concurrent swapon will reuse this swap_info while its previous
> > > resources are not cleared completely.
> > > 
> > > These late freed resources are:
> > >  - p->percpu_cluster
> > >  - swap_cgroup_ctrl[type]
> > >  - block_device setting
> > >  - inode->i_flags &= ~S_SWAPFILE
> > > 
> > > This patch clear SWP_USED flag after all its resources freed, so that swapon
> > > can reuse this swap_info by alloc_swap_info() safely.
> > > 
> > > This patch is just for a rare scenario, aim to correct of code.
> > 
> > I believe that
> > http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch
> > makes this patch redundant?
> > 
> 
> oop, hang on.  This patch *is* a stealth-updated version of
> http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch.
> 
> Undocumented removals of si->swap_map have been added.  What's going on
> there?
> 
> I think I'll stick with the original patch for now.  If you see
> additional optimisations or changes, let's address that separately?

Correct decision, thanks: I explained in an answer when Acking the
previous version why I dislike this version (it would prevent you
from watching the slow progress of swapoff).

Hugh
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