Re: page fault scalability patch V12 [0/7]: Overview and performance tests

From: Hugh Dickins
Date: Sun Dec 12 2004 - 04:34:53 EST

On Sun, 12 Dec 2004, Nick Piggin wrote:
> Christoph Lameter wrote:
> > On Thu, 9 Dec 2004, Hugh Dickins wrote:
> >>probably others (harder to think through). Your 4/7 patch for i386 has
> >>an unused atomic get_64bit function from Nick, I think you'll have to
> >>define a get_pte_atomic macro and use get_64bit in its 64-on-32 cases.
> >
> > That would be a performance issue.
> Problems were pretty trivial to reproduce here with non atomic 64-bit
> loads being cut in half by atomic 64 bit stores. I don't see a way
> around them, unfortunately.

Of course, it'll only be a performance issue in the 64-on-32 cases:
the 64-on-64 and 32-on-32 macro should reduce to exactly the present
"entry = *pte".

I've had the impression that Christoph and SGI have to care a great
deal more about ia64 than the others; and as x86_64 advances, so
i386 PAE grows less important. Just so long as a get_64bit there
isn't a serious degradation from present behaviour, it's okay.

Oh, hold on, isn't handle_mm_fault's pmd without page_table_lock
similarly racy, in both the 64-on-32 cases, and on architectures
which have a more complex pmd_t (sparc, m68k, h8300)? Sigh.


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