Re: [PATCH V2] mm: Introduce GFP_PGTABLE

From: Matthew Wilcox
Date: Wed Jan 16 2019 - 07:30:28 EST

On Wed, Jan 16, 2019 at 07:57:03AM +0100, Michal Hocko wrote:
> On Wed 16-01-19 11:51:32, Anshuman Khandual wrote:
> > All architectures have been defining their own PGALLOC_GFP as (GFP_KERNEL |
> > __GFP_ZERO) and using it for allocating page table pages. This causes some
> > code duplication which can be easily avoided. GFP_KERNEL allocated and
> > cleared out pages (__GFP_ZERO) are required for page tables on any given
> > architecture. This creates a new generic GFP flag flag which can be used
> > for any page table page allocation. Does not cause any functional change.
> >
> > GFP_PGTABLE is being added into include/asm-generic/pgtable.h which is the
> > generic page tabe header just to prevent it's potential misuse as a general
> > allocation flag if included in include/linux/gfp.h.
> I haven't reviewed the patch yet but I am wondering whether this is
> really worth it without going all the way down to unify the common code
> and remove much more code duplication. Or is this not possible for some
> reason?

Exactly what I suggested doing in response to v1.

Also, the approach taken here is crazy. x86 has a feature that no other
architecture has bothered to implement yet -- accounting page tables
to the process. Yet instead of spreading that goodness to all other
architectures, Anshuman has gone to more effort to avoid doing that.