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

[Joonsoo Kim]

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.

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.