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

[David Hildenbrand]

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