linux-kernel - Re: [PATCH] [8/16] HWPOISON: Use bitmask/action code for try_to

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Date:	Wed, 10 Jun 2009 08:07:54 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Lee.Schermerhorn@...com" <Lee.Schermerhorn@...com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] [8/16] HWPOISON: Use bitmask/action code for try_to_unmap behaviour

On Wed, Jun 10, 2009 at 10:27:36AM +0800, Wu Fengguang wrote:
> On Tue, Jun 09, 2009 at 05:57:25PM +0800, Nick Piggin wrote:
> > On Wed, Jun 03, 2009 at 08:46:41PM +0200, Andi Kleen wrote:
> > > 
> > > try_to_unmap currently has multiple modi (migration, munlock, normal unmap)
> > > which are selected by magic flag variables. The logic is not very straight
> > > forward, because each of these flag change multiple behaviours (e.g.
> > > migration turns off aging, not only sets up migration ptes etc.)
> > > Also the different flags interact in magic ways.
> > > 
> > > A later patch in this series adds another mode to try_to_unmap, so 
> > > this becomes quickly unmanageable.
> > > 
> > > Replace the different flags with a action code (migration, munlock, munmap)
> > > and some additional flags as modifiers (ignore mlock, ignore aging).
> > > This makes the logic more straight forward and allows easier extension
> > > to new behaviours. Change all the caller to declare what they want to 
> > > do.
> > > 
> > > This patch is supposed to be a nop in behaviour. If anyone can prove 
> > > it is not that would be a bug.
> > > 
> > > Cc: Lee.Schermerhorn@...com
> > > Cc: npiggin@...e.de
> > > 
> > > Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> > > 
> > > ---
> > >  include/linux/rmap.h |   14 +++++++++++++-
> > >  mm/migrate.c         |    2 +-
> > >  mm/rmap.c            |   40 ++++++++++++++++++++++------------------
> > >  mm/vmscan.c          |    2 +-
> > >  4 files changed, 37 insertions(+), 21 deletions(-)
> > > 
> > > Index: linux/include/linux/rmap.h
> > > ===================================================================
> > > --- linux.orig/include/linux/rmap.h	2009-06-03 19:36:23.000000000 +0200
> > > +++ linux/include/linux/rmap.h	2009-06-03 20:39:50.000000000 +0200
> > > @@ -84,7 +84,19 @@
> > >   * Called from mm/vmscan.c to handle paging out
> > >   */
> > >  int page_referenced(struct page *, int is_locked, struct mem_cgroup *cnt);
> > > -int try_to_unmap(struct page *, int ignore_refs);
> > > +
> > > +enum ttu_flags {
> > > +	TTU_UNMAP = 0,			/* unmap mode */
> > > +	TTU_MIGRATION = 1,		/* migration mode */
> > > +	TTU_MUNLOCK = 2,		/* munlock mode */
> > > +	TTU_ACTION_MASK = 0xff,
> > > +
> > > +	TTU_IGNORE_MLOCK = (1 << 8),	/* ignore mlock */
> > > +	TTU_IGNORE_ACCESS = (1 << 9),	/* don't age */
> > > +};
> > > +#define TTU_ACTION(x) ((x) & TTU_ACTION_MASK)
> > 
> > I still think this is nasty and should work like Gfp flags.
> 
> I don't see big problems here.
> 
> We have page_zone() and gfp_zone(), so why not TTU_ACTION()? We could
> allocate one bit for each action code, but in principle they are exclusive.

I haven't actually applied the patchset and looked at the resulting
try_to_unmap function, so I'll hold my tounge until I do that. I'll
send a patch if I can find something that looks nicer. So don't worry
about it for the moment.


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