Re: [PATCH 1/2] mm/swap: Fix race conditions in swap_slots cache init

From: Andrew Morton
Date: Tue Oct 03 2017 - 18:27:44 EST

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