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Subject: Re: [RFC PATCH 0/8] mm/madvise: support
 process_madvise(MADV_DONTNEED)
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <YVG2DJx9t6FGr4kX@dhcp22.suse.cz>
Date:   Mon, 27 Sep 2021 12:12:46 -0700
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>
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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>
 <YVG2DJx9t6FGr4kX@dhcp22.suse.cz>
To:     Michal Hocko <mhocko@suse.com>
Precedence: bulk


> On Sep 27, 2021, at 5:16 AM, Michal Hocko <mhocko@suse.com> wrote:
>=20
> 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:
>>=20
>> 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 =E2=80=9Cswap=E2=80=9D.
>> 4. Calls process_madvise(MADV_DONTNEED) to zap it.
>=20
> Why cannot you use MADV_PAGEOUT/MADV_COLD for this usecase?

Providing hints to the kernel takes you so far to a certain extent.
The kernel does not want to (for a good reason) to be completely
configurable when it comes to reclaim and prefetch policies. Doing
so from userspace allows you to be fully configurable.

> 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.

I would argue that there is nothing bad that remote MADV_DONTNEED can do
that process_vm_writev() cannot do as well (putting aside ptrace).

process_vm_writev() is checking:

	mm =3D mm_access(task, PTRACE_MODE_ATTACH_REALCREDS)

Wouldn't adding such a condition suffice?=