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

[Nick Piggin]

Hugh Dickins wrote:
> 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.

I was more thinking of atomic pte reads there. I had for some reason
thought that dup_mmap only had a down_read of the mmap_sem. But even if
it did only down_read, a further look showed this wouldn't have been a
problem for Christoph anyway. That dim light-bulb probably changes things
for my patches too; I may be able to do copy_page_range with fewer atomics.

I'm still not too sure that all places read the pte atomically where needed.
But presently this is not a really big concern because it only would
really slow down i386 PAE if anything.

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