On 06.01.19 00:31, 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.
>
> 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.
>
> [mhocko@xxxxxxxx: provide a clearer changelog for the last two paragraph]
>
> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>
> ---
> v4:
> * adjust last two paragraph changelog from Michal's comment
> v3:
> * it is not proper to rely on caller to drain pages, so keep to drain
> pages during iteration and remove the one in callers.
> v2: adjust changelog with MIGRATE_ISOLATE effects for the isolated range
> ---
> mm/memory_hotplug.c | 1 -
> mm/page_alloc.c | 1 -
> 2 files changed, 2 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 6910e0eea074..d2fa6cbbb2db 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1599,7 +1599,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>
> cond_resched();
> lru_add_drain_all();
> - drain_all_pages(zone);
>
> pfn = scan_movable_pages(start_pfn, end_pfn);
> if (pfn) { /* We have movable pages */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f1edd36a1e2b..d9ee4bb3a1a7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8041,7 +8041,6 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> */
>
> lru_add_drain_all();
> - drain_all_pages(cc.zone);
>
> order = 0;
> outer_start = start;
>
--
Thanks,
David / dhildenb
