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Date: Tue, 3 Oct 2017 15:27:37 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Ying Huang <ying.huang@intel.com>,
        Wenwei Tao <wenwei.tww@alibaba-inc.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Minchan Kim <minchan@kernel.org>,
        Rik van Riel <riel@redhat.com>, Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>,
        Hillf Danton <hillf.zj@alibaba-inc.com>
Subject: Re: [PATCH 1/2] mm/swap: Fix race conditions in swap_slots cache
 init
Message-Id: <20171003152737.c955053c04ee6ad9f70dc5eb@linux-foundation.org>
In-Reply-To: <65a9d0f133f63e66bba37b53b2fd0464b7cae771.1500677066.git.tim.c.chen@linux.intel.com>
References: <65a9d0f133f63e66bba37b53b2fd0464b7cae771.1500677066.git.tim.c.chen@linux.intel.com>
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On Fri, 21 Jul 2017 15:45:00 -0700 Tim Chen <tim.c.chen@linux.intel.com> wrote:

> Memory allocations can happen before the swap_slots cache initialization
> is completed during cpu bring up.  If we are low on memory, we could call
> get_swap_page and access swap_slots_cache before it is fully initialized.
> 
> Add a check in get_swap_page for initialized swap_slots_cache
> to prevent this condition.  Similar check already exists in
> free_swap_slot.  Also annotate the checks to indicate the likely
> condition.
> 
> We also added a memory barrier to make sure that the locks
> initialization are done before the assignment of cache->slots
> and cache->slots_ret pointers. This ensures the assumption
> that it is safe to acquire the slots cache locks and use the slots
> cache when the corresponding cache->slots or cache->slots_ret
> pointers are non null.

I guess that the user-visible effect is "crash on boot on large
machine".  Or something.  Please don't make me guess!

Which kernel version(s) do you believe need this patch, and why?