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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <nst3wauaphvvnkseuatqknxfhtu5ewf7zqmoskim5kt52wf2mi@sasls2f6r22i>
Date: Mon, 7 Oct 2024 12:01:32 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Anthony Yznaga <anthony.yznaga@...cle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, 
	markhemm@...glemail.com, viro@...iv.linux.org.uk, david@...hat.com, 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 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.

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

> 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov