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

["Huang\, Ying" <ying.huang@intel.com>]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Thu, 14 Dec 2017 16:17:18 +0100 Michal Hocko <mhocko@kernel.org> 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>

Although I still thought SRCU based solution is better, I will prepare a
version with preempt_disable() + stop_machine() or rcu_read_lock() +
synchronize_rcu() based version for people to compare between them.

BTW, it appears that rcu_read_lock() + synchronize_rcu() is better than
preempt_disable() + stop_machine(), why not use it?

Best Regards,
Huang, Ying

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>