Re: [PATCH 1/2] mm: refactor reinsert of swap_info in sys_swapoff

[Konrad Rzeszutek Wilk]

On Tue, Oct 30, 2012 at 02:04:17PM -0700, Andrew Morton wrote:
> On Sat, 27 Oct 2012 19:20:46 -0200
> Cesar Eduardo Barros <cesarb@xxxxxxxxxx> wrote:
> 
> > The block within sys_swapoff which re-inserts the swap_info into the
> > swap_list in case of failure of try_to_unuse() reads a few values outside
> > the swap_lock. While this is safe at that point, it is subtle code.
> > 
> > Simplify the code by moving the reading of these values to a separate
> > function, refactoring it a bit so they are read from within the
> > swap_lock. This is easier to understand, and matches better the way it
> > worked before I unified the insertion of the swap_info from both
> > sys_swapon and sys_swapoff.
> > 
> > This change should make no functional difference. The only real change
> > is moving the read of two or three structure fields to within the lock
> > (frontswap_map_get() is nothing more than a read of p->frontswap_map).
> 
> Your patch doesn't change this, but...  it is very unusual for any
> subsystem's ->init method to be called under a spinlock.  Because it is
> highly likely that such a method will wish to do things such as memory
> allocation.
> 
> It is rare and unlikely for an ->init() method to *need* such external
> locking, because all the objects it is dealing with cannot be looked up
> by other threads because nothing has been registered anywhere yet.

I don't believe it actually needs that locking. Dan, do you recall
the details of this?
> 
> So either frontswap is doing something wrong here or there's some
> subtlety which escapes me.  If the former then we should try to get
> that ->init call to happen outside swap_lock.

Agreed.
> 
> And if we can do that, perhaps we can fix the regrettable GFP_ATOMIC
> in zcache_new_pool().

Ouch. Yes.


FYI, thanks for pulling those two patches - they looked good to me
but I hadn't had a chance to test them so did not want to comment on them
until that happen. Dan beat me to it and he did test them.

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