Re: [PATCH 1/2] mm: drop migrate type checks from has_unmovable_pages

[Joonsoo Kim]

On Fri, Oct 20, 2017 at 10:17:00AM +0200, Michal Hocko wrote:
> On Fri 20-10-17 15:22:14, Xishi Qiu wrote:
> > On 2017/10/20 10:13, Joonsoo Kim wrote:
> > 
> > > On Thu, Oct 19, 2017 at 02:21:18PM +0200, Michal Hocko wrote:
> [...]
> > >> >From 8cbd811d741f5dd93d1b21bb3ef94482a4d0bd32 Mon Sep 17 00:00:00 2001
> > >> From: Michal Hocko <mhocko@xxxxxxxx>
> > >> Date: Thu, 19 Oct 2017 14:14:02 +0200
> > >> Subject: [PATCH] mm: distinguish CMA and MOVABLE isolation in
> > >>  has_unmovable_pages
> > >>
> > >> Joonsoo has noticed that "mm: drop migrate type checks from
> > >> has_unmovable_pages" would break CMA allocator because it relies on
> > >> has_unmovable_pages returning false even for CMA pageblocks which in
> > >> fact don't have to be movable:
> > >> alloc_contig_range
> > >>   start_isolate_page_range
> > >>     set_migratetype_isolate
> > >>       has_unmovable_pages
> > >>
> > >> This is a result of the code sharing between CMA and memory hotplug
> > >> while each one has a different idea of what has_unmovable_pages should
> > >> return. This is unfortunate but fixing it properly would require a lot
> > >> of code duplication.
> > >>
> > >> Fix the issue by introducing the requested migrate type argument
> > >> and special case MIGRATE_CMA case where CMA page blocks are handled
> > >> properly. This will work for memory hotplug because it requires
> > >> MIGRATE_MOVABLE.
> > > 
> > > Unfortunately, alloc_contig_range() can be called with
> > > MIGRATE_MOVABLE so this patch cannot perfectly fix the problem.
> > > 
> > > I did a more thinking and found that it's strange to check if there is
> > > unmovable page in the pageblock during the set_migratetype_isolate().
> > > set_migratetype_isolate() should be just for setting the migratetype
> > > of the pageblock. Checking other things should be done by another
> > > place, for example, before calling the start_isolate_page_range() in
> > > __offline_pages().
> > > 
> > > Thanks.
> > > 
> > 
> > Hi Joonsoo,
> > 
> > How about add a flag to skip or not has_unmovable_pages() in set_migratetype_isolate()?
> > Something like the skip_hwpoisoned_pages.
> 
> I believe this is what Joonsoo was proposing actually. I cannot say I

Yes, I initially suggested this idea but change my mind. Now, I think
that problem is not in has_unmovable_pages() but in
set_migratetype_isolate(). So different solution is needed. See my other
reply to Michal.

Thanks.