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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        Zi Yan <ziy@nvidia.com>, Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Minchan Kim <minchan@kernel.org>,
        "Johannes Weiner" <hannes@cmpxchg.org>,
        Hugh Dickins <hughd@google.com>,
        "Alexander Duyck" <alexander.duyck@gmail.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating
References: <20200228033819.3857058-1-ying.huang@intel.com>
        <20200228034248.GE29971@bombadil.infradead.org>
        <87a7538977.fsf@yhuang-dev.intel.com>
        <edae2736-3239-0bdc-499c-560fc234c974@redhat.com>
        <871rqf850z.fsf@yhuang-dev.intel.com>
        <20200228095048.GK3771@dhcp22.suse.cz>
        <87d09u7sm2.fsf@yhuang-dev.intel.com>
        <20200302142549.GO4380@dhcp22.suse.cz>
        <874kv66x8r.fsf@yhuang-dev.intel.com>
        <20200303081929.GY4380@dhcp22.suse.cz>
Date:   Tue, 03 Mar 2020 19:36:57 +0800
In-Reply-To: <20200303081929.GY4380@dhcp22.suse.cz> (Michal Hocko's message of
        "Tue, 3 Mar 2020 09:19:29 +0100")
Message-ID: <87k1414qli.fsf@yhuang-dev.intel.com>
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Michal Hocko <mhocko@kernel.org> writes:

> On Tue 03-03-20 09:30:28, Huang, Ying wrote:
> [...]
>> Yes.  mmap() can control whether to populate the underlying physical
>> pages.
>
> right because many usecases benefit from it. They simply know that the
> mapping will be used completely and it is worth saving overhead for #PF.
> See. there is a clear justification for that policy.
>
>> But for migrating MADV_FREE pages, there's no control, all pages
>> will be populated again always by default.  Maybe we should avoid to do
>> that in some situations too.
>
> Now let's have a look here. It is the userspace that decided to mark
> MADV_FREE pages. It is under its full control which pages are to be
> freed lazily. If the userspace wants to move those pages then it is
> likely aware they have been MADV_FREE, right? If the userspace wanted to
> save migration overhead then it could either chose to not migrate those
> pages or simply unmap them right away. So in the end we are talking
> about saving munmap/MAMDV_DONTNEED or potentially more move_pages calls
> to skip over MADV_FREE holes. Which is all nice but is there any
> userspace that really does care? Because this is a fundamental question
> here and it doesn't make much sense to discuss this left to right unless
> this is clear.

Although I don't agree with you, I don't want to continue.  Because I
feel that the discussion may be too general to go anywhere.  I admit
that I go to the general side firstly, sorry about that.

Best Regards,
Huang, Ying