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

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Dec 19, 2018 at 10:57:19AM +0100, Oscar Salvador 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().
>
>I had the same fear, but then I saw that move_freepages_block()->move_freepages() moves
>the pages to a new list:
>
><--
>list_move(&page->lru,
>			  &zone->free_area[order].free_list[migratetype]);
>-->
>
>
>But looking at it again, I see that this is only for BuddyPages, so I guess
>that pcp-pages do not really get unlinked, so we could still allocate them.

Well, I think you are right. But with this in mind, current code looks
buggy.

Between has_unmovable_pages() and drain_all_pages(), others still could
allocate pages on pcp list, right? This means we thought we have
isolated the range, but not.

So even we do drain_all_pages(), we still missed some pages in this
range.

>
>Uhmf, I missed that.
>
>-- 
>Oscar Salvador
>SUSE L3

-- 
Wei Yang
Help you, Help me