Re: [RFC] Demand faulting for large pages

[Andi Kleen <ak@suse.de>]

On Fri, Aug 05, 2005 at 11:37:27AM -0500, Adam Litke wrote:
> 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?

It's one of the four numa policies: DEFAULT, PREFERED, INTERLEAVE, BIND

It just means allocate on the local node if possible, otherwise fall back.

You said you wanted INTERLEAVE by default, which i think is a bad idea.
It should be only optional like in all other allocations.


> > > 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.

The problem is that get_user_pages needs to handle the case of a large
page not yet being faulted in properly. The SLES9 implementation did
some changes for this.

You don't change it at all, so I'm suspect it doesn't work yet.

It's a common case - think people doing raw IO on huge pages shared memory.

-Andi
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