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Date:   Mon, 27 Sep 2021 14:16:12 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Minchan Kim <minchan@kernel.org>,
        Colin Cross <ccross@google.com>,
        Suren Baghdasarya <surenb@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/8] mm/madvise: support
 process_madvise(MADV_DONTNEED)
Message-ID: <YVG2DJx9t6FGr4kX@dhcp22.suse.cz>
References: <20210926161259.238054-1-namit@vmware.com>
 <7ce823c8-cfbf-cc59-9fc7-9aa3a79740c3@redhat.com>
 <6E8A03DD-175F-4A21-BCD7-383D61344521@gmail.com>
 <2753a311-4d5f-8bc5-ce6f-10063e3c6167@redhat.com>
 <AE756194-07D4-4467-92CA-9E986140D85D@gmail.com>
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On Mon 27-09-21 05:00:11, Nadav Amit wrote:
[...]
> The manager is notified on memory regions that it should monitor
> (through PTRACE/LD_PRELOAD/explicit-API). It then monitors these regions
> using the remote-userfaultfd that you saw on the second thread. When it wants
> to reclaim (anonymous) memory, it:
> 
> 1. Uses UFFD-WP to protect that memory (and for this matter I got a vectored
>    UFFD-WP to do so efficiently, a patch which I did not send yet).
> 2. Calls process_vm_readv() to read that memory of that process.
> 3. Write it back to “swap”.
> 4. Calls process_madvise(MADV_DONTNEED) to zap it.

Why cannot you use MADV_PAGEOUT/MADV_COLD for this usecase?

MADV_DONTNEED on a remote process has been proposed in the past several
times and it has always been rejected because it is a free ticket to all
sorts of hard to debug problems as it is just a free ticket for a remote
memory corruption. An additional capability requirement might reduce the
risk to some degree but I still do not think this is a good idea.
-- 
Michal Hocko
SUSE Labs