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

[Michal Hocko]

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
would like skip_hwpoisoned_pages and adding one more flag is just too
ugly. So I would prefer to have something that would actually make sense
from the semantic POV. If CMA really needs to work with partial CMA
blocks then MIGRATE_CMA as an indicator sounds like the right way to go
to me. But I am not a CMA expert so I might be missing some subtlety
here.
-- 
Michal Hocko
SUSE Labs