On Thu, Nov 27, 2008 at 11:22:57AM -0800, Mike Waychison wrote:Nick Piggin wrote:On Thu, Nov 27, 2008 at 11:00:07AM +0100, Peter Zijlstra wrote:Right, this can be an issue. The way around it should be to minimize the length of time any single lock holder can sit on it. Compared to what we have today with:
pagemap_read looks like it can use get_user_pages_fast. The smaps and
clear_refs stuff might have been nicer if they could work on ranges
like pagemap. Then they could avoid mmap_sem as well (although maps
would need to be sampled and take mmap_sem I guess).
One problem with dropping mmap_sem is that it hurts priority/fairness.
And it opens a bit of a (maybe theoretical but not something to completely
ignore) forward progress hole AFAIKS. If mmap_sem is very heavily
contended, then the refault is going to take a while to get through,
and then the page might get reclaimed etc).
- sleep in major fault with read lock held,
- enqueue writer behind it,
- and make all other faults wait on the rwsem
The retry logic seems to be a lot better for forward progress.
The whole reason why you have the latency is because it is
guaranteeing forward progress for everyone. The retry logic
may work out better in that situation, but it does actually
open a starvation hole.