linux-kernel - frontswap: is frontswap_init called from swapoff safe?

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Date:	Tue, 05 Jun 2012 07:41:52 -0300
From:	Cesar Eduardo Barros <cesarb@...arb.net>
To:	linux-kernel@...r.kernel.org
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	linux-mm@...ck.org
Subject: frontswap: is frontswap_init called from swapoff safe?

I was looking at the swapfile.c parts of the recently-merged frontswap, 
and noticed that frontswap_init can be called from swapoff when 
try_to_unuse fails.

This looks odd to me. Whether it is safe or not depends on what 
frontswap_ops.init does, but the comment for __frontswap_init ("Called 
when a swap device is swapon'd") and the function name itself seem to 
imply it should be called only for swapon, not when relinking the 
swap_info after a failed swapoff.

In particular, if frontswap_ops.init assumes the swap map is empty, it 
would break, since as far as I know when try_to_unuse fails there are 
still pages in the swap.

(By the way, the comment above enable_swap_info at sys_swapoff needs to 
be updated to also explain why reading p->frontswap_map outside the lock 
is safe at that point, like it does for p->prio and p->swap_map.)

-- 
Cesar Eduardo Barros
cesarb@...arb.net
cesar.barros@...il.com
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