Re: [PATCH v3] mm: remove extra drain pages on pcp list

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

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@gmail.com>
>
>With something like that, you can add
>Acked-by: Michal Hocko <mhocko@suse.com>
>

Thanks, would adjust it accordingly.

-- 
Wei Yang
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