Re: [RFC] Demand faulting for large pages

[Adam Litke <agl@us.ibm.com>]

On Fri, 2005-08-05 at 10:53, Andi Kleen wrote:
> On Fri, Aug 05, 2005 at 10:21:38AM -0500, Adam Litke wrote:
> > Below is a patch to implement demand faulting for huge pages.  The main
> > motivation for changing from prefaulting to demand faulting is so that
> > huge page allocations can follow the NUMA API.  Currently, huge pages
> > are allocated round-robin from all NUMA nodes.   
> 
> I think matching DEFAULT is better than having a different default for
> huge pages than for small pages.

I am not exactly sure what the above means.  Is 'DEFAULT' a system
default numa allocation policy?

> In general more programs are happy with local memory than remote memory.

I totally agree.

> Also it makes it consistent.
> 
> > 
> > The default behavior in SLES9 for i386 is to use demand faulting with
> > NUMA policy-aware allocations.  To my knowledge, this continues to work
> 
> Not sure what you're trying to say here. All allocations are NUMA policy aware.

Sorry, I really wasn't clear.  That statement referred to huge pages
specifically.  I was trying to point out that numa policy-aware huge
page allocation combined with demand faulting in SLES9/i386 has been a
success.

> > well in practice.  Thanks to consolidated hugetlb code, switching the
> > behavior requires changing only one fault handler.  The bulk of the
> > patch just moves the logic from hugelb_prefault() to
> > hugetlb_pte_fault().
> 
> Are you sure you fixed get_user_pages to handle this properly? It doesn't
> like it.

Unless I am missing something, the call to follow_hugetlb_page() in
get_user_pages() is just an optimization.  Removing it means
follow_page() will be called individually for each PAGE_SIZE page in the
huge page.  We can probably do better but I didn't want to cloud this
patch with that logic.

-- 
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center