linux-kernel - Re: [RFC PATCH 0/8] mm/madvise: support process_madvise(MADV

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Date:   Mon, 27 Sep 2021 12:12:46 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        Colin Cross <ccross@...gle.com>,
        Suren Baghdasarya <surenb@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/8] mm/madvise: support
 process_madvise(MADV_DONTNEED)


> On Sep 27, 2021, at 5:16 AM, Michal Hocko <mhocko@...e.com> wrote:
> 
> 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?

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 = mm_access(task, PTRACE_MODE_ATTACH_REALCREDS)

Wouldn't adding such a condition suffice?