linux-kernel - Re: [PATCH 18 of 66] add pmd mangling functions to x86

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Date:	Mon, 29 Nov 2010 10:23:11 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 18 of 66] add pmd mangling functions to x86

On Fri, Nov 26, 2010 at 06:57:51PM +0100, Andrea Arcangeli wrote:
> On Thu, Nov 18, 2010 at 01:04:46PM +0000, Mel Gorman wrote:
> > On Wed, Nov 03, 2010 at 04:27:53PM +0100, Andrea Arcangeli wrote:
> > > From: Andrea Arcangeli <aarcange@...hat.com>
> > > 
> > > Add needed pmd mangling functions with simmetry with their pte counterparts.
> > 
> > symmetry
> 
> Fixed.
> 
> > 
> > > pmdp_freeze_flush is the only exception only present on the pmd side and it's
> > > needed to serialize the VM against split_huge_page, it simply atomically clears
> > > the present bit in the same way pmdp_clear_flush_young atomically clears the
> > > accessed bit (and both need to flush the tlb to make it effective, which is
> > > mandatory to happen synchronously for pmdp_freeze_flush).
> > 
> > I don't see a pmdp_freeze_flush defined in the patch. Did yu mean 
> > pmdp_splitting_flush? Even if it is, it's the splitting bit you are
> > dealing with which isn't the same as the present bit. I'm missing
> > something.
> 
> Well the comment went out of sync with the code sorry. I updated it:
> 
> =======
> Add needed pmd mangling functions with symmetry with their pte counterparts.
> pmdp_splitting_flush() is the only new addition on the pmd_ methods and it's
> needed to serialize the VM against split_huge_page. It simply atomically sets
> the splitting bit in a similar way pmdp_clear_flush_young atomically clears the
> accessed bit. pmdp_splitting_flush() also has to flush the tlb to make it
> effective against gup_fast, but it wouldn't really require to flush the tlb
> too. Just the tlb flush is the simplest operation we can invoke to serialize
> pmdp_splitting_flush() against gup_fast.
> =======
> 

Much clearer, thanks.

> > > Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> > > Acked-by: Rik van Riel <riel@...hat.com>
> > > ---
> > > 
> > > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> > > --- a/arch/x86/include/asm/pgtable.h
> > > +++ b/arch/x86/include/asm/pgtable.h
> > > @@ -302,15 +302,15 @@ pmd_t *populate_extra_pmd(unsigned long 
> > >  pte_t *populate_extra_pte(unsigned long vaddr);
> > >  #endif	/* __ASSEMBLY__ */
> > >  
> > > +#ifndef __ASSEMBLY__
> > > +#include <linux/mm_types.h>
> > > +
> > >  #ifdef CONFIG_X86_32
> > >  # include "pgtable_32.h"
> > >  #else
> > >  # include "pgtable_64.h"
> > >  #endif
> > >  
> > > -#ifndef __ASSEMBLY__
> > > -#include <linux/mm_types.h>
> > > -
> > 
> > Stupid quetion: Why is this move necessary?
> 
> That's not a stupid question, it seems to build in all configurations
> even with this part backed out. I'll try to reverse this one in the
> hope that it won't break build. I suppose some earlier version of the
> patchset required this to build (I would never make a gratuitous
> change like this if it wasn't needed at some point) but it seems not
> be required anymore according to my build tests. If I'm wrong and some
> build breaks I'll reintroduce it later.
> 

Ok.

> > >  static inline int pte_none(pte_t pte)
> > >  {
> > >  	return !pte.pte;
> > > @@ -353,7 +353,7 @@ static inline unsigned long pmd_page_vad
> > >   * Currently stuck as a macro due to indirect forward reference to
> > >   * linux/mmzone.h's __section_mem_map_addr() definition:
> > >   */
> > > -#define pmd_page(pmd)	pfn_to_page(pmd_val(pmd) >> PAGE_SHIFT)
> > > +#define pmd_page(pmd)	pfn_to_page((pmd_val(pmd) & PTE_PFN_MASK) >> PAGE_SHIFT)
> > >  
> > 
> > Why is it now necessary to use PTE_PFN_MASK?
> 
> Just for the NX bit, that couldn't be set before the pmd could be
> marked PSE.
> 

Sorry, I still am missing something. PTE_PFN_MASK is this

#define PTE_PFN_MASK            ((pteval_t)PHYSICAL_PAGE_MASK)
#define PHYSICAL_PAGE_MASK      (((signed long)PAGE_MASK) & __PHYSICAL_MASK)

I'm not seeing how PTE_PFN_MASK affects the NX bit (bit 63).

> > The implementations look fine but I'm having trouble reconsiling what
> > the leader says with the patch :(
> 
> Yes because it was out of sync, the new version is above.
> 

Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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