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From:   Zi Yan <ziy@nvidia.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     Vlastimil Babka <vbabka@suse.cz>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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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 09:59:00 -0800
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On 18 Feb 2019, at 9:52, Matthew Wilcox wrote:

> On Mon, Feb 18, 2019 at 09:51:33AM -0800, Zi Yan wrote:
>> 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.
>
> You allocate one temporary page per thread that's doing an exchange_page().

Yeah, you are right. I think at most I need NR_CPU order-0 pages. I will try
it. Thanks.

--
Best Regards,
Yan Zi

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