linux-kernel - Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes

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Message-ID: <d080442f-be33-474b-9c4a-bdb57d14cd2c@redhat.com>
Date: Mon, 7 Oct 2024 21:41:11 +0200
From: David Hildenbrand <david@...hat.com>
To: Anthony Yznaga <anthony.yznaga@...cle.com>,
 "Kirill A. Shutemov" <kirill@...temov.name>
Cc: akpm@...ux-foundation.org, willy@...radead.org, markhemm@...glemail.com,
 viro@...iv.linux.org.uk, khalid@...nel.org, andreyknvl@...il.com,
 dave.hansen@...el.com, luto@...nel.org, brauner@...nel.org, arnd@...db.de,
 ebiederm@...ssion.com, catalin.marinas@....com, linux-arch@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhiramat@...nel.org,
 rostedt@...dmis.org, vasily.averin@...ux.dev, xhao@...ux.alibaba.com,
 pcc@...gle.com, neilb@...e.de, maz@...nel.org
Subject: Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes

On 07.10.24 21:23, Anthony Yznaga wrote:
> 
> On 10/7/24 2:01 AM, Kirill A. Shutemov wrote:
>> On Tue, Sep 03, 2024 at 04:22:31PM -0700, Anthony Yznaga wrote:
>>> This patch series implements a mechanism that allows userspace
>>> processes to opt into sharing PTEs. It adds a new in-memory
>>> filesystem - msharefs. A file created on msharefs represents a
>>> shared region where all processes mapping that region will map
>>> objects within it with shared PTEs. When the file is created,
>>> a new host mm struct is created to hold the shared page tables
>>> and vmas for objects later mapped into the shared region. This
>>> host mm struct is associated with the file and not with a task.
>> Taskless mm_struct can be problematic. Like, we don't have access to it's
>> counters because it is not represented in /proc. For instance, there's no
>> way to check its smaps.
> 
> Definitely needs exposure in /proc. One of the things I'm looking into
> is the feasibility of showing the mappings in maps/smaps/etc..
> 
> 
>>
>> Also, I *think* it is immune to oom-killer because oom-killer looks for a
>> victim task, not mm.
>> I hope it is not an intended feature :P
> 
> oom-killer would have to kill all sharers of an mshare region before the
> mshare region itself could be freed, but I'm not sure that oom-killer
> would be the one to free the region. An mshare region is essentially a
> shared memory object not unlike a tmpfs or hugetlb file. I think some
> higher level intelligence would have to be involved to release it if
> appropriate when under oom conditions.

I thought we discussed that at LSF/MM last year and the conclusion was 
that a process is needed (OOM kill, cgroup handling, ...), and it must 
remain running. Once it stops running, we can force-unmap all pages etc.

Did you look at the summary/(recording if available) of that, or am I 
remembering things wrongly or was there actually such a discussion?

I know, it's problematic that this feature switched owners, ...

-- 
Cheers,

David / dhildenb