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From:   Zi Yan <ziy@nvidia.com>
To:     Vlastimil Babka <vbabka@suse.cz>
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Subject: Re: [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange two
 lists of pages.
Date:   Mon, 18 Feb 2019 09:51:33 -0800
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On 18 Feb 2019, at 9:42, Vlastimil Babka wrote:

> On 2/18/19 6:31 PM, Zi Yan wrote:
>> The purpose of proposing exchange_pages() is to avoid allocating any 
>> new
>> page,
>> so that we would not trigger any potential page reclaim or memory
>> compaction.
>> Allocating a temporary page defeats the purpose.
>
> Compaction can only happen for order > 0 temporary pages. Even if you 
> used
> single order = 0 page to gradually exchange e.g. a THP, it should be 
> better than
> u64. Allocating order = 0 should be a non-issue. If it's an issue, 
> then the
> system is in a bad state and physically contiguous layout is a 
> secondary concern.

You are right if we only need to allocate one order-0 page. But this 
also means
we can only exchange two pages at a time. We need to add a lock to make 
sure
the temporary page is used exclusively or we need to keep allocating 
temporary pages
when multiple exchange_pages() are happening at the same time.

--
Best Regards,
Yan Zi