Re: Re: Splitting the mmap_sem

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 09, 2020 at 12:13:20PM -0800, Matthew Wilcox wrote:
> One of the use cases that we're concerned about involves a high
> percentage of page faults on a single large (terabytes) VMA (and a
> highly multithreaded process).  Moving the contention from a rwsem
> in the mm_struct to a refcount in the VMA will not help performance
> substantially for this user.

This is why I never believed in the VMA-refcount approach, and why my
patches used SRCU.

> The proposal consists of three phases.  In phase 1, we convert the
> rbtree to the maple tree, and leave the locking alone.  In phase 2,
> we change the locking to a per-VMA refcount, looked up under RCU.
> 
> This problem arises during phase 3 where we attempt to handle page
> faults entirely under the RCU read lock.  If we encounter problems,
> we can fall back to acquiring the VMA refcount, but we need the
> page allocation to fail rather than sleep (or magically drop the
> RCU lock and return an indication that it has done so, but that
> doesn't seem to be an approach that would find any favour).

So why not use SRCU? You can do full blocking faults under SRCU and
don't need no 'stinkin' refcounts ;-)