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Date:   Mon, 18 Feb 2019 09:52:24 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Zi Yan <ziy@nvidia.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>, 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>
Subject: Re: [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange
 two lists of pages.
Message-ID: <20190218175224.GT12668@bombadil.infradead.org>
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>
 <2630a452-8c53-f109-1748-36b98076c86e@suse.cz>
 <53690FCD-B0BA-4619-8DF1-B9D721EE1208@nvidia.com>
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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().