Re: page table lock patch V15 [0/7]: overview

[Hugh Dickins]

On Thu, 13 Jan 2005, Nick Piggin wrote:
> Andrew Morton wrote:
> 
> Note that this was with my ptl removal patches. I can't see why Christoph's
> would have _any_ extra overhead as they are, but it looks to me like they're
> lacking in atomic ops. So I'd expect something similar for Christoph's when
> they're properly atomic.
> 
> > Look, -7% on a 2-way versus +700% on a many-way might well be a tradeoff we
> > agree to take.  But we need to fully understand all the costs and benefits.
> 
> I think copy_page_range is the one to keep an eye on.

Christoph's currently lack set_pte_atomics in the fault handlers, yes.
But I don't see why they should need set_pte_atomics in copy_page_range
(which is why I persuaded him to drop forcing set_pte to atomic).

dup_mmap has down_write of the src mmap_sem, keeping out any faults on
that.  copy_pte_range has spin_lock of the dst page_table_lock and the
src page_table_lock, keeping swapout away from those.  Why would atomic
set_ptes be needed there?  Probably in yours, but not in Christoph's.

Hugh

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