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Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
To:     Michal Hocko <mhocko@suse.com>
Cc:     Peter Xu <peterx@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Minchan Kim <minchan@kernel.org>, Jann Horn <jannh@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dave Hansen <dave.hansen@intel.com>,
        Hugh Dickins <hughd@google.com>,
        Rik van Riel <riel@surriel.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
        Helge Deller <deller@gmx.de>, Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>, linux-alpha@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
        linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org
References: <20210217154844.12392-1-david@redhat.com>
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 <b24996a6-7652-f88c-301e-28417637fd02@redhat.com>
 <YDOnq9Nliopj9kQL@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <296a1a02-f7ec-5085-f17e-eadc4bdb6a24@redhat.com>
Date:   Mon, 22 Feb 2021 13:52:16 +0100
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On 22.02.21 13:46, Michal Hocko wrote:
> I am slowly catching up with this thread.
> 
> On Fri 19-02-21 09:20:16, David Hildenbrand wrote:
> [...]
>> So if we have zero, we write zero. We'll COW pages, triggering a write fault
>> - and that's the only good thing about it. For example, similar to
>> MADV_POPULATE, nothing stops KSM from merging anonymous pages again. So for
>> anonymous memory the actual write is not helpful at all. Similarly for
>> hugetlbfs, the actual write is not necessary - but there is no other way to
>> really achieve the goal.
> 
> I really do not see why you care about KSM so much. Isn't KSM an
> explicit opt-in with a fine grained interface to control which memory to
> KSM or not?

Yeah, I think it's opt-in via MADV_MERGEABLE. E.g., QEMU defaults to 
enable KSM unless explicitly disabled by the user.

But I agree, I got distracted by KSM details.

-- 
Thanks,

David / dhildenb