linux-kernel - Re: [PATCH v2 0/9] Add support for shared PTEs across processes

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Message-ID: <0864a811-53c8-a87b-a32d-d6f4c7945caa@redhat.com>
Date:   Fri, 8 Jul 2022 13:47:46 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Khalid Aziz <khalid.aziz@...cle.com>
Cc:     willy@...radead.org, aneesh.kumar@...ux.ibm.com, arnd@...db.de,
        21cnbao@...il.com, corbet@....net, dave.hansen@...ux.intel.com,
        ebiederm@...ssion.com, hagen@...u.net, jack@...e.cz,
        keescook@...omium.org, kirill@...temov.name, kucharsk@...il.com,
        linkinjeon@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        longpeng2@...wei.com, luto@...nel.org, markhemm@...glemail.com,
        pcc@...gle.com, rppt@...nel.org, sieberf@...zon.com,
        sjpark@...zon.de, surenb@...gle.com, tst@...oebel-theuer.de,
        yzaikin@...gle.com
Subject: Re: [PATCH v2 0/9] Add support for shared PTEs across processes

On 02.07.22 06:24, Andrew Morton wrote:
> On Wed, 29 Jun 2022 16:53:51 -0600 Khalid Aziz <khalid.aziz@...cle.com> wrote:
> 
>> This patch series implements a mechanism in kernel to allow
>> userspace processes to opt into sharing PTEs. It adds a new
>> in-memory filesystem - msharefs. 
> 
> Dumb question: why do we need a new filesystem for this?  Is it not
> feasible to permit PTE sharing for mmaps of tmpfs/xfs/ext4/etc files?
> 

IIRC, the general opinion at LSF/MM was that this approach at hand is
makes people nervous and I at least am not convinced that we really want
to have this upstream.

What's *completely* missing from the cover letter are the dirty details:
"Actual data is mmap'ed using anonymous pages, ext4/xfs/btfrfs/etc
files.". Gah.

As raised already, "anonymous pages" makes me shiver.

(To me, what I read, this looks like an RFC to me, yet I see "v2". But I
am a little confused why most of the feedback at LSF/MM seems to be
ignored and people are moving forward with this approach. But maybe my
memory is wrong.)

Please, let's look into more generic page table sharing just like
hugetlb already provides to some degree. And if we need additional
alignment requirements to share multiple page table levels, then let's
look into that as well for special users.

-- 
Thanks,

David / dhildenb