Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY

[Nick Piggin]

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:
> 
> >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).
> 
> 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:
> 
>   - 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.

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