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

[Michal Hocko <mhocko@suse.com>]

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
prevent start_isolate_page_range to work on multiple zones? At least
contiguous allocator can do that in principle.

So no I do not like this patch, it is not an improvement.
-- 
Michal Hocko
SUSE Labs