Re: [PATCH 1/2] numa, mm: drop redundant check indo_huge_pmd_numa_page()

[Kirill A. Shutemov]

On Fri, Oct 26, 2012 at 04:07:44PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-10-26 at 16:57 +0300, Kirill A. Shutemov wrote:
> > > > Yes, this code will catch it:
> > > > 
> > > >     /* if an huge pmd materialized from under us just retry later */
> > > >     if (unlikely(pmd_trans_huge(*pmd)))
> > > >             return 0;
> > > > 
> > > > If the pmd is under splitting it's still a pmd_trans_huge().
> > > 
> > > OK, so then we simply keep taking the same fault until the split is
> > > complete? Wouldn't it be better to wait for it instead of spin on
> > > faults?
> > 
> > IIUC, on next fault we will wait split the page in fallow_page(). 
> 
> What follow_page()?, a regular hardware page-fault will not call
> follow_page() afaict, we do a down_read(), find_vma() and call
> handle_mm_fault() -- with a lot of error and corner case checking in
> between.

Yeah, you're right. Then, it seems we're spinning on the fault until the
page is splitted.

I'm not sure how long spliting takes and if splitting itself can fix some
fault reason.

-- 
 Kirill A. Shutemov

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