Re: [PATCH v2] mm, page_isolation: remove drain_all_pages() in set_migratetype_isolate()

[Michal Hocko <mhocko@suse.com>]

On Wed 19-12-18 13:29:34, Wei Yang wrote:
> On Wed, Dec 19, 2018 at 10:51:10AM +0100, Michal Hocko wrote:
> >On Wed 19-12-18 04:46:56, Wei Yang wrote:
> >> Below is a brief call flow for __offline_pages() and
> >> alloc_contig_range():
> >> 
> >>   __offline_pages()/alloc_contig_range()
> >>       start_isolate_page_range()
> >>           set_migratetype_isolate()
> >>               drain_all_pages()
> >>       drain_all_pages()
> >> 
> >> Current logic is: isolate and drain pcp list for each pageblock and
> >> drain pcp list again. This is not necessary and we could just drain pcp
> >> list once after isolate this whole range.
> >> 
> >> The reason is start_isolate_page_range() will set the migrate type of
> >> a range to MIGRATE_ISOLATE. After doing so, this range will never be
> >> allocated from Buddy, neither to a real user nor to pcp list.
> >
> >But it is important to note that those pages still can be allocated from
> >the pcp lists until we do drain_all_pages().
> >
> >One thing that I really do not like about this patch (and I believe I
> >have mentioned that previously) that you rely on callers to do the right
> >thing. The proper fix would be to do the draining in
> >start_isolate_page_range and remove them from callers. Also what does
> 
> Well, I don't really understand this meaning previously.
> 
> So you prefer set_migratetype_isolate() do the drain instead of the
> caller (__offline_pages) do the drain. Is my understanding correct?

Either set_migratetype_isolate or start_isolate_page_range. The later
only if this is guaranteed that we cannot intemix zones in the range.

> >prevent start_isolate_page_range to work on multiple zones? At least
> >contiguous allocator can do that in principle.
> 
> As the comment mentioned, in current implementation the range must be in
> one zone.

I do not see anything like that documented for set_migratetype_isolate.
-- 
Michal Hocko
SUSE Labs