Re: [PATCH] Make is_mem_section_removable more conformable with offlining code

[Michal Hocko <mhocko@suse.cz>]

Hi,

On Mon 23-08-10 11:22:46, Michal Hocko wrote:
> On Sun 22-08-10 08:42:32, Wu Fengguang wrote:
> > Hi Michal,
> 
> Hi,
> 
> > 
> > It helps to explain in changelog/code
> > 
> > - in what situation a ZONE_MOVABLE will contain !MIGRATE_MOVABLE
> >   pages? 
> 
> page can be MIGRATE_RESERVE IIUC.
> 
> >   And why the MIGRATE_MOVABLE test is still necessary given the
> >   ZONE_MOVABLE check?
> 
> I would assume that the MIGRATE_MOVABLE test is not necessary (given that
> the whole zone is set as movable) but this test is used also in the
> offlining path (in set_migratetype_isolate) and the primary reason for
> this patch is to sync those two checks. 
> 
> I am not familiar with all the possible cases for migrate flags so the
> test reduction should be better done by someone more familiar with the
> code (the zone flag test is much more easier than the whole
> get_pageblock_migratetype so this could be a win in the end).
> 
> > 
> > - why do you think free pages are not removeable? Simply to cater for
> >   the set_migratetype_isolate() logic, or there are more fundamental
> >   reasons?
> 
> Free pages can be from non movable zone, right? I know that having a
> zone with the free page blocks in non-movable zone is extremely 
> improbable but what is the point of this check anyway? So yes, this is
> more to be in sync than anything more fundamental.

Are there any other comments on this? Is the patch reasonable at all?

> 
> > 
> > Thanks,
> > Fengguang
> > 
> > On Fri, Aug 20, 2010 at 04:14:00PM +0200, Michal Hocko wrote:
> > > Hi,
> > > what do you think about the patch below?
> > > 
> > > >From b983695b92b5be58f31c719fada1d3245f7b6768 Mon Sep 17 00:00:00 2001
> > > From: Michal Hocko <mhocko@suse.cz>
> > > Date: Fri, 20 Aug 2010 15:39:16 +0200
> > > Subject: [PATCH] Make is_mem_section_removable more conformable with offlining code
> > > 
> > > Currently is_mem_section_removable checks whether each pageblock from
> > > the given pfn range is of MIGRATE_MOVABLE type or if it is free. If both
> > > are false then the range is considered non removable.
> > > 
> > > On the other hand, offlining code (more specifically
> > > set_migratetype_isolate) doesn't care whether a page is free and instead
> > > it just checks the migrate type of the page and whether the page's zone
> > > is movable.
> > > 
> > > This can lead into a situation when a node is marked as removable even
> > > though all pages are neither MIGRATE_MOVABLE nor the zone is
> > > ZONE_MOVABLE.
> > > 
> > > Also we can mark a node as not removable just because a pageblock is
> > > MIGRATE_RESERVE and not free (and this situation is much more probable).
> > > ---
> > >  mm/memory_hotplug.c |    6 +++---
> > >  1 files changed, 3 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > > index a4cfcdc..da20568 100644
> > > --- a/mm/memory_hotplug.c
> > > +++ b/mm/memory_hotplug.c
> > > @@ -611,10 +611,10 @@ int is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
> > >  		type = get_pageblock_migratetype(page);
> > >  
> > >  		/*
> > > -		 * A pageblock containing MOVABLE or free pages is considered
> > > -		 * removable
> > > +		 * A pageblock containing MOVABLE or page from movable
> > > +		 * zone are considered removable
> > >  		 */
> > > -		if (type != MIGRATE_MOVABLE && !pageblock_free(page))
> > > +		if (type != MIGRATE_MOVABLE && zone_idx(page) != ZONE_MOVABLE)
> > >  			return 0;
> > >  
> > >  		/*

-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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