Re: [RFC 2/2] Hugetlb COW

[David Gibson <david@gibson.dropbear.id.au>]

On Mon, Nov 07, 2005 at 03:47:55PM -0600, Adam Litke wrote:
> On Mon, 2005-11-07 at 15:38 -0600, Adam Litke wrote:
> > [RFC] COW for hugepages
> > (Patch originally from David Gibson <dwg@au1.ibm.com>)
> > 
> > This patch implements copy-on-write for hugepages, hence allowing
> > MAP_PRIVATE mappings of hugetlbfs.
> > 
> > This is chiefly useful for cases where we want to use hugepages
> > "automatically" - that is to map hugepages without the knowledge of
> > the code in the final application (either via kernel hooks, or with
> > LD_PRELOAD).  We can use various heuristics to determine when
> > hugepages might be a good idea, but changing the semantics of
> > anonymous memory from MAP_PRIVATE to MAP_SHARED without the app's
> > knowledge is clearly wrong.
> 
> I forgot to mention in the original post that this patch is currently
> broken on ppc64 due to a problem with update_mmu_cache().  The proper
> fix is understood but backed up behind the powerpc merge activity.  

Actually, BenH just pointed out that the version of update_mmu_cache()
we now have from the 64k patch already half-handled this situation -
it wouldn't actually do anything on a hugepage fault, but it wouldn't
blow up either.  So the COW patch should work as is on ppc64, although
the update_mmu_cache change is probably an important optimization
(otherwise we'll take two faults on every COW - one to actually do the
COW, and one to twiddle the bits in the hash page table).

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
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