Re: [PATCH] Make is_mem_section_removable more conformable withofflining code

[Michal Hocko]

On Thu 02-09-10 20:19:45, Hiroyuki Kamezawa wrote:
> 2010/9/2 Michal Hocko <mhocko@xxxxxxx>:
> > On Thu 02-09-10 18:03:43, KAMEZAWA Hiroyuki wrote:
> >> On Thu, 2 Sep 2010 10:28:29 +0200
> >> Michal Hocko <mhocko@xxxxxxx> wrote:
> >>
> >> > On Thu 02-09-10 14:45:00, KAMEZAWA Hiroyuki wrote:
[...]
> >> > By the higher fragmentation you mean that all movable pageblocks (even
> >> > reclaimable) gets to MIGRATE_MOVABLE until we get first failure. In the
> >> > worst case, if we fail near the end of the zone then there is imbalance
> >> > in MIGRATE_MOVABLE vs. MIGRATE_RECALIMABLE. Is that what you are
> >> > thinking of? Doesn't this just gets the zone to the state after
> >> > onlining? Or is the problem if we fail somewhere in the middle?
> >> >
> >>
> >> No. My concern is pageblock type changes before/after memory hotplug failure.
> >> ? ? ? before isolation: MIGRATE_RECLAIMABLE
> >> ? ? ? after isolation failure : MIGRATE_MOVABLE
> >
> > Ahh, OK I can see your point now. unset_migratetype_isolate called on
> > the failure path sets migrate type unconditionally as it cannot know
> > what was the original migration type.
> >
> Right.
> 
> > What about MIGRATE_RESERVE? Is there anything that can make those
> > allocations fail offlining?
> >
> MIGRATE_RESERVE can contain several typs of pages, mixture of movable/unmovable
> pages.

Ahh, ok. This is just a fallback zone. I see.

> 
> IIRC, my 1st version of code of set_migratetype_isolate() just checks
> zone_idx and
> I think checking MIGRATE_TYPE is my mistake.
> (As Mel explained, it can be a mixture of several types.)
> 
> So, how about using the latter half of set_migratetype_isolate()'s check ?
> It checks that the given range just includes free pages and LRU pages.
> It's 100% accurate and more trustable than migrate_type check.
> 
> Whatever migratetype the pageblock has, if the block only contains free pages
> and lru pages, changing the type as MOVABLE (at failure) is not very bad.
> 
> (Or, checking contents of pageblock in failure path and set proper
> MIGRATE type.)
> 
> Anyway, not very difficult. Just a bit larger patch than you have.

What about this? Just compile tested.

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