linux-kernel - Re: [PATCH 1/2] mm/swap: Fix race conditions in swap

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Date:   Tue, 3 Oct 2017 15:27:37 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Ying Huang <ying.huang@...el.com>,
        Wenwei Tao <wenwei.tww@...baba-inc.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>
Subject: Re: [PATCH 1/2] mm/swap: Fix race conditions in swap_slots cache
 init

On Fri, 21 Jul 2017 15:45:00 -0700 Tim Chen <tim.c.chen@...ux.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?