Re: [PATCH V2] mm: Introduce GFP_PGTABLE

[Christophe Leroy]

Le 16/01/2019 Ã 14:18, Matthew Wilcox a ÃcritÂ:
> On Wed, Jan 16, 2019 at 06:42:22PM +0530, Anshuman Khandual wrote:
> > On 01/16/2019 06:00 PM, Matthew Wilcox wrote:
> > > 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.
> >
> > The basic objective for this patch is to create a common minimum allocation
> > flag that can be used by architectures but that still allows archs to add
> > on additional constraints if they see fit. This patch does not intend to
> > change functionality for any arch.
>
> I disagree with your objective.  Making more code common is a great idea,
> but this patch is too unambitious.  We should be heading towards one or
> two page table allocation functions instead of having every architecture do
> its own thing.
>
> So start there.  Move the x86 function into common code and convert one
> other architecture to use it too.

Are we talking about pte_alloc_one_kernel() and pte_alloc_one() ?

I'm not sure x86 function is the best common one, as it seems to 
allocate a multiple of PAGE_SIZE only.

Some arches like powerpc use pagetables which are smaller than a page, 
for instance powerpc 8xx uses 4k pagetables even with 16k pages, which 
means a single page can be used by 4 pagetables.

Therefore, I would suggest to start with powerpc functions.

Christophe