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

[Joonsoo Kim]

On Thu, Oct 19, 2017 at 09:15:03AM +0200, Michal Hocko wrote:
> On Thu 19-10-17 11:51:11, Joonsoo Kim wrote:
> > On Fri, Oct 13, 2017 at 02:00:12PM +0200, Michal Hocko wrote:
> > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > 
> > > Michael has noticed that the memory offline tries to migrate kernel code
> > > pages when doing
> > >  echo 0 > /sys/devices/system/memory/memory0/online
> > > 
> > > The current implementation will fail the operation after several failed
> > > page migration attempts but we shouldn't even attempt to migrate
> > > that memory and fail right away because this memory is clearly not
> > > migrateable. This will become a real problem when we drop the retry loop
> > > counter resp. timeout.
> > > 
> > > The real problem is in has_unmovable_pages in fact. We should fail if
> > > there are any non migrateable pages in the area. In orther to guarantee
> > > that remove the migrate type checks because MIGRATE_MOVABLE is not
> > > guaranteed to contain only migrateable pages. It is merely a heuristic.
> > > Similarly MIGRATE_CMA does guarantee that the page allocator doesn't
> > > allocate any non-migrateable pages from the block but CMA allocations
> > > themselves are unlikely to migrateable. Therefore remove both checks.
> > 
> > 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()

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.

Thanks.