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

[Michal Hocko]

On Fri 20-10-17 11:13:29, Joonsoo Kim wrote:
> On Thu, Oct 19, 2017 at 02:21:18PM +0200, Michal Hocko wrote:
> > On Thu 19-10-17 10:20:41, Michal Hocko wrote:
> > > On Thu 19-10-17 16:33:56, Joonsoo Kim wrote:
> > > > On Thu, Oct 19, 2017 at 09:15:03AM +0200, Michal Hocko wrote:
> > > > > On Thu 19-10-17 11:51:11, Joonsoo Kim wrote:
> > > [...]
> > > > > > Hello,
> > > > > > 
> > > > > > This patch will break the CMA user. As you mentioned, CMA allocation
> > > > > > itself isn't migrateable. So, after a single page is allocated through
> > > > > > CMA allocation, has_unmovable_pages() will return true for this
> > > > > > pageblock. Then, futher CMA allocation request to this pageblock will
> > > > > > fail because it requires isolating the pageblock.
> > > > > 
> > > > > Hmm, does this mean that the CMA allocation path depends on
> > > > > has_unmovable_pages to return false here even though the memory is not
> > > > > movable? This sounds really strange to me and kind of abuse of this
> > > > 
> > > > Your understanding is correct. Perhaps, abuse or wrong function name.
> > > >
> > > > > function. Which path is that? Can we do the migrate type test theres?
> > > > 
> > > > alloc_contig_range() -> start_isolate_page_range() ->
> > > > set_migratetype_isolate() -> has_unmovable_pages()
> > > 
> > > I see. It seems that the CMA and memory hotplug have a very different
> > > view on what should happen during isolation.
> > >  
> > > > We can add one argument, 'XXX' to set_migratetype_isolate() and change
> > > > it to check migrate type rather than has_unmovable_pages() if 'XXX' is
> > > > specified.
> > > 
> > > Can we use the migratetype argument and do the special thing for
> > > MIGRATE_CMA? Like the following diff?
> > 
> > And with the full changelog.
> > ---
> > >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.

Yes, alloc_contig_range can be called with MIGRATE_MOVABLE but my
understanding is that only CMA allocator really depends on this weird
semantic and that does MIGRATE_CMA unconditionally.

> 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().

How do we guarantee the atomicity?
-- 
Michal Hocko
SUSE Labs