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Date:   Wed, 16 Jan 2019 13:44:31 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>, linux-mm@kvack.org,
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Subject: Re: [PATCH V2] mm: Introduce GFP_PGTABLE
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On Wed 16-01-19 04:30:18, 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.

Yes, I believe the only reason this is x86 only is that each arch would
have to be tweaked separately. So a cleanup in _that_ regard would be
helpful. There is no real reason to have ptes accounted only for x86.
There might be some exceptions but well, our asm-generic allows to opt
in for generic implementation or override it with a special one. The
later should be an exception rather than the rule.
-- 
Michal Hocko
SUSE Labs