On Thu, Jan 03, 2019 at 02:56:09PM +0100, Michal Hocko wrote: >On Sat 22-12-18 01:02:28, Wei Yang wrote: >> In current implementation, there are two places to isolate a range of >> page: __offline_pages() and alloc_contig_range(). During this procedure, >> it will drain pages on pcp list. >> >> Below is a brief call flow: >> >> __offline_pages()/alloc_contig_range() >> start_isolate_page_range() >> set_migratetype_isolate() >> drain_all_pages() >> drain_all_pages() <--- A >> >> >From this snippet we can see current logic is isolate and drain pcp list >> for each pageblock and drain pcp list again for the whole range. >> >> While the drain at A is not necessary. 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. This means the procedure >> to drain pages on pcp list after start_isolate_page_range() will not >> drain any page in the target range. > >I am still not happy with the changelog. I would suggest the following >instead > >" >start_isolate_page_range is responsible for isolating the given pfn >range. One part of that job is to make sure that also pages that are on >the allocator pcp lists are properly isolated. Otherwise they could be >reused and the range wouldn't be completely isolated until the memory is >freed back. While there is no strict guarantee here because pages might >get allocated at any time before drain_all_pages is called there doesn't >seem to be any strong demand for such a guarantee. > >In any case, draining is already done at the isolation level and there >is no need to do it again later by start_isolate_page_range callers >(memory hotplug and CMA allocator currently). Therefore remove pointless >draining in existing callers to make the code more clear and >functionally correct. >" > >> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx> > >With something like that, you can add >Acked-by: Michal Hocko <mhocko@xxxxxxxx> > Thanks, would adjust it accordingly. -- Wei Yang Help you, Help me
