Re: [RFC][PATCH 0/6] Another go at speculative page faults

[Peter Zijlstra]

On Mon, Oct 20, 2014 at 05:07:02PM -0700, Andy Lutomirski wrote:
> On 10/20/2014 02:56 PM, Peter Zijlstra wrote:
> > Hi,
> > 
> > I figured I'd give my 2010 speculative fault series another spin:
> > 
> >   https://lkml.org/lkml/2010/1/4/257
> > 
> > Since then I think many of the outstanding issues have changed sufficiently to
> > warrant another go. In particular Al Viro's delayed fput seems to have made it
> > entirely 'normal' to delay fput(). Lai Jiangshan's SRCU rewrite provided us
> > with call_srcu() and my preemptible mmu_gather removed the TLB flushes from
> > under the PTL.
> > 
> > The code needs way more attention but builds a kernel and runs the
> > micro-benchmark so I figured I'd post it before sinking more time into it.
> > 
> > I realize the micro-bench is about as good as it gets for this series and not
> > very realistic otherwise, but I think it does show the potential benefit the
> > approach has.
> 
> Does this mean that an entire fault can complete without ever taking
> mmap_sem at all?  If so, that's a *huge* win.

Yep.

> I'm a bit concerned about drivers that assume that the vma is unchanged
> during .fault processing.  In particular, is there a race between .close
> and .fault?  Would it make sense to add a per-vma rw lock and hold it
> during vma modification and .fault calls?

VMA granularity contention would be about as bad as mmap_sem for many
workloads. But yes, that is one of the things we need to look at, I was
_hoping_ that holding the file open would sort most these problems, but
I'm sure there plenty 'interesting' cruft left.
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