linux-kernel - Re: [PATCH v1 00/14] Add support for shared PTEs across processes

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Message-ID: <YlR8kDs3I1jx6Oxs@casper.infradead.org>
Date:   Mon, 11 Apr 2022 20:08:00 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>, akpm@...ux-foundation.org,
        aneesh.kumar@...ux.ibm.com, arnd@...db.de, 21cnbao@...il.com,
        corbet@....net, dave.hansen@...ux.intel.com, david@...hat.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 v1 00/14] Add support for shared PTEs across processes

On Mon, Apr 11, 2022 at 11:51:46AM -0700, Dave Hansen wrote:
> On 4/11/22 10:37, Matthew Wilcox wrote:
> > Another argument that MM developers find compelling is that we can reduce
> > some of the complexity in hugetlbfs where it has the ability to share
> > page tables between processes.
> 
> When could this complexity reduction actually happen in practice?  Can
> this mshare thingy be somehow dropped in underneath the existing
> hugetlbfs implementation?  Or would userspace need to change?

Userspace needs to opt in to mshare, so there's going to be a transition
period where we still need hugetlbfs to still support it, but I have
the impression that the users that need page table sharing are pretty
specialised and we'll be able to find them all before disabling it.

I don't think we can make it transparent to userspace, but I'll noodle
on that a bit.