linux-kernel - Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

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Date:   Sat, 22 Jan 2022 16:09:13 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Thomas Schoebel-Theuer <tst@...oebel-theuer.de>
Cc:     "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>, Khalid Aziz <khalid.aziz@...cle.com>,
        Barry Song <21cnbao@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

On Sat, Jan 22, 2022 at 11:18:14AM +0100, Thomas Schoebel-Theuer wrote:
> On 1/22/22 2:41 AM, Matthew Wilcox wrote:
> > On Sat, Jan 22, 2022 at 01:39:46AM +0000, Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote:
> > > > > Our use case is that we have some very large files stored on persistent
> > > > > memory which we want to mmap in thousands of processes.  So the first
> > > The memory overhead of PTEs would be significantly saved if we use
> > > hugetlbfs in this case, but why not?
> > Because we want the files to be persistent across reboots.
> 
> 100% agree. There is another use case: geo-redundancy.
> 
> My view is publicly documented at
> https://github.com/schoebel/mars/tree/master/docu and click at
> architecture-guide-geo-redundancy.pdf

That's a 160+ page PDF.  No offence, Thomas, I'm not reading that to
try to understand how you want to use page table sharing.

> In some scenarios, migration or (temporary) co-existence of block devices
> from/between hardware architecture A to/between hardware architecture B
> might become a future requirement for me.

I'm not sure how sharing block devices between systems matches up with
sharing page tables between processes.

> It would be great if msharefs is not only low-footprint, but also would be
> usable from kernelspace.

I don't understand what you want here either.  Kernel threads already
share their page tables.