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Date:   Mon, 14 Dec 2020 15:19:46 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, vbabka@suse.cz, david@redhat.com,
        osalvador@suse.de, dan.j.williams@intel.com, sashal@kernel.org,
        tyhicks@linux.microsoft.com, iamjoonsoo.kim@lge.com,
        mike.kravetz@oracle.com, rostedt@goodmis.org, mingo@redhat.com,
        jgg@ziepe.ca, peterz@infradead.org, mgorman@suse.de,
        willy@infradead.org, rientjes@google.com, jhubbard@nvidia.com,
        linux-doc@vger.kernel.org
Subject: Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone
Message-ID: <20201214141946.GG32193@dhcp22.suse.cz>
References: <20201211202140.396852-1-pasha.tatashin@soleen.com>
 <20201211202140.396852-6-pasha.tatashin@soleen.com>
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On Fri 11-12-20 15:21:39, Pavel Tatashin wrote:
[...]
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index b593316bff3d..25c0c13ba4b1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -386,9 +386,14 @@ enum zone_type {
>  	 * likely to succeed, and to locally limit unmovable allocations - e.g.,
>  	 * to increase the number of THP/huge pages. Notable special cases are:
>  	 *
> -	 * 1. Pinned pages: (long-term) pinning of movable pages might
> -	 *    essentially turn such pages unmovable. Memory offlining might
> -	 *    retry a long time.
> +	 * 1. Pinned pages: (long-term) pinning of movable pages is avoided
> +	 *    when pages are pinned and faulted, but it is still possible that
> +	 *    address space already has pages in ZONE_MOVABLE at the time when
> +	 *    pages are pinned (i.e. user has touches that memory before
> +	 *    pinning). In such case we try to migrate them to a different zone,
> +	 *    but if migration fails the pages can still end-up pinned in
> +	 *    ZONE_MOVABLE. In such case, memory offlining might retry a long
> +	 *    time and will only succeed once user application unpins pages.

I do agree with others in the thread. This is not really helping out the
current situation much. You should simply fail the pin rather than
pretend all is just fine.
-- 
Michal Hocko
SUSE Labs