linux-kernel - Re: [PATCH v6 08/14] mm/gup: do not migrate zero page

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Message-ID: <20210125142819.GU4605@ziepe.ca>
Date:   Mon, 25 Jan 2021 10:28:19 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 08/14] mm/gup: do not migrate zero page

On Wed, Jan 20, 2021 at 09:26:41AM -0500, Pavel Tatashin wrote:

> I thought about this, and it would code a little cleaner. But, the
> reason I did not is because zero_page is perfectly pinnable, it is not
> pinnable only when it is in a movable zone (and it should not be in a
> movable zones for other reasons as well), but that is another bug that
> needs to be resolved, and once that bug is resolved this condition can
> be removed from gup migration.

My point is you've defined the zero page to be pinnable no matter what
zone it is in, so is_pinnable(zero_page) == true

Jason