linux-kernel - Re: + x86-add-support-for-pud-sized-transparent-hugepages-checkpatch-fixes.patch added to -mm tree

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Date:	Wed, 9 Mar 2016 13:45:40 -0500
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>, akpm@...ux-foundation.org,
	hpa@...or.com, mingo@...e.hu, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: +
 x86-add-support-for-pud-sized-transparent-hugepages-checkpatch-fixes.patch
 added to -mm tree

On Wed, Mar 09, 2016 at 06:40:09PM +0100, Andrea Arcangeli wrote:
> On Fri, Mar 04, 2016 at 03:30:18PM -0500, Matthew Wilcox wrote:
> > I have no idea what it means.  This is copy-and-change of the pmd version,
> > which was originally commit db3eb96f4e6281b84dd33c8980dacc27f2efe177 by
> > Andrea.
> 
> which I also copied from native_ptep_get_and_clear:

Hah ;-)

> The comment means native_local_pmdp_get_and_clear() couldn't be
> called, or the build would break because of preprocessor include order
> dependencies. I CC'ed Jeremy just in case, but I've no doubts about
> the comment myself.
> 
> See also what native_local_pmdp_get_and_clear does..
> 
> static inline pmd_t native_local_pmdp_get_and_clear(pmd_t *pmdp)
> {
> 	pmd_t res = *pmdp;
> 
> 	native_pmd_clear(pmdp);
> 	return res;
> }
> 
> It'd be sure fine to improve the comment, but a comment, even a short
> one, was in order. If a solution is found for the include ordering,
> one could call native_local_pmdp_get_and_clear there, so it was good
> to keep that in mind. Nothing special about the pmd-THP part, this
> build issue originated in the pte.
> 
> In fact even before starting to fix the comment, I would recommend to
> try again to call native_local_pmdp_get_and_clear and
> native_local_ptep_get_and_clear to verify if it still breaks, just in
> case the include ordering got fixed by accident in the meanwhile (that
> was a comment in 2.6.25 when arch/x86/include/asm didn't even exist
> yet, it was still in include/asm-x86). If it would manage to build
> without the manual expansion, the comment could go and the duplication
> as well.

The ordering problem is still there.  native_local_ptep_get_and_clear()
is declared at line 726 of asm/pgtable.h and asm/pgtable_64.h is included
at line 466 of asm/pgtable.h.

I'll have a little play; see if I can resolve this ...