Re: [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup

On Fri 03-06-16 17:10:01, Andrea Arcangeli wrote:

Hello Michal,

CC'ed Hugh,

On Fri, Jun 03, 2016 at 04:46:00PM +0200, Michal Hocko wrote:

quoted

What do you think about the external dependencies mentioned above. Do
you think this is a sufficient argument wrt. occasional higher
latencies?

It's a tradeoff and both latencies would be short and uncommon so it's
hard to tell.

There's also mmput_async for paths that may care about mmput
latencies. Exit itself cannot use it, it's mostly for people taking
the mm_users pin that may not want to wait for mmput to run. It also
shouldn't happen that often, it's a slow path.

The whole model inherited from KSM is to deliberately depend only on
the mmap_sem + test_exit + mm_count, and never on mm_users, which to
me in principle doesn't sound bad.

I do agree that this model is quite clever (albeit convoluted). It just
assumes that all other mmap_sem users are behaving the same. Now most
in-kernel users will do get_task_mm() and then lock mmap_sem, but I
haven't checked all of them and it is quite possible that some of those
would like to optimize in a similar way and only increment mm_count.
I might be too pessimistic about the out of mm code but I would feel
much better if the exit path didn't depend on them.

Anyway, if the current model sounds better I will definitely not insist
on my patch. It is more of an idea for simplification than a fix for
anything I have seen happening in the real life.
-- 
Michal Hocko
SUSE Labs