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Subject: Re: [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange two
 lists of pages.
To:     Zi Yan <ziy@nvidia.com>, Matthew Wilcox <willy@infradead.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Michal Hocko <mhocko@kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        John Hubbard <jhubbard@nvidia.com>,
        Mark Hairgrove <mhairgrove@nvidia.com>,
        Nitin Gupta <nigupta@nvidia.com>,
        David Nellans <dnellans@nvidia.com>
References: <20190215220856.29749-1-zi.yan@sent.com>
 <20190215220856.29749-2-zi.yan@sent.com>
 <20190217112943.GP12668@bombadil.infradead.org>
 <65A1FFA0-531C-4078-9704-3F44819C3C07@nvidia.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <2630a452-8c53-f109-1748-36b98076c86e@suse.cz>
Date:   Mon, 18 Feb 2019 18:42:59 +0100
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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.