Re: [RFC 2/2] Hugetlb COW

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

On Mon, Nov 07, 2005 at 03:35:38PM -0800, William Lee Irwin wrote:
> On Mon, Nov 07, 2005 at 03:38:53PM -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'll go check for architectures where page protections may be encoded
> differently depending on the size of the translation, or whose code is
> otherwise unprepared to cope with protection bits.
> 
> If you've done such checking already, I'd be much obliged to hear of it
> (in fact, I'd much prefer you to have done so).

I can't see how the COW catch could be any more broken in this regard
than we are already:  make_huge_pte() in mm/hugetlb.c already assumes
that pte_mkwrite() and pte_wrprotect() will work properly on hugepage
PTEs.  COW doesn't use anything more.

-- 
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_!
http://www.ozlabs.org/~dgibson