frontswap: is frontswap_init called from swapoff safe?

[Cesar Eduardo Barros <cesarb@cesarb.net>]

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@cesarb.net
cesar.barros@gmail.com

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