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

[Anthony Yznaga]

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.


> > When a process mmap's the shared region, a vm flag VM_SHARED_PT
> > is added to the vma. On page fault the vma is checked for the
> > presence of the VM_SHARED_PT flag.
> I think it is wrong approach.
>
> Instead of spaying VM_SHARED_PT checks across core-mm, we need to add a
> generic hooks that can be used by mshare and hugetlb. And remove
> is_vm_hugetlb_page() check from core-mm along the way.
>
> BTW, is_vm_hugetlb_page() callsites seem to be the indicator to check if
> mshare has to do something differently there. I feel you miss a lot of
> such cases.

Good point about is_vm_hugetlb_page(). I'll review the callsites (there 
are only ~60 of them :-).


Thanks,

Anthony