linux-kernel - Re: [PATCH RFC] mm/madvise: introduce MADV

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Date:   Mon, 22 Feb 2021 13:52:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>, Jann Horn <jannh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dave Hansen <dave.hansen@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, linux-alpha@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory

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