Re: [PATCH -mm -V2] mm, swap: Fix race between swapoff and some swap operations

[Andrew Morton]

On Thu, 14 Dec 2017 16:17:18 +0100 Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> > as fast as possible, SRCU instead of reference count is used to
> > implement get/put_swap_device().  From get_swap_device() to
> > put_swap_device(), the reader side of SRCU is locked, so
> > synchronize_srcu() in swapoff() will wait until put_swap_device() is
> > called.
> 
> It is quite unfortunate to pull SRCU as a dependency to the core kernel.
> Different attempts to do this have failed in the past. This one is
> slightly different though because I would suspect that those tiny
> systems do not configure swap. But who knows, maybe they do.
> 
> Anyway, if you are worried about performance then I would expect some
> numbers to back that worry. So why don't simply start with simpler
> ref count based and then optimize it later based on some actual numbers.
> Btw. have you considered pcp refcount framework. I would suspect that
> this would give you close to SRCU performance.

<squeaky-wheel>Or use stop_kernel() ;)</squeaky-wheel>