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

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Message-ID: <YfFW43PT5hwPHnOz@casper.infradead.org>
Date:   Wed, 26 Jan 2022 14:12:51 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        akpm@...ux-foundation.org, longpeng2@...wei.com, arnd@...db.de,
        dave.hansen@...ux.intel.com, rppt@...nel.org, surenb@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Peter Xu <peterx@...hat.com>
Subject: Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

On Wed, Jan 26, 2022 at 02:55:10PM +0100, David Hildenbrand wrote:
> On 26.01.22 14:38, Matthew Wilcox wrote:
> > On Wed, Jan 26, 2022 at 11:16:42AM +0100, David Hildenbrand wrote:
> >> A while ago I talked with Peter about an extended uffd (here: WP)
> >> mechanism that would work on fds instead of the process address space.
> > 
> > As far as I can tell, uffd is a grotesque hack that exists to work around
> > the poor choice to use anonymous memory instead of file-backed memory
> > in kvm.  Every time I see somebody mention it, I feel pain.
> > 
> 
> I might be missing something important, because KVM can deal with
> file-back memory just fine and uffd is used heavily outside of hypervisors.
> 
> I'd love to learn how to handle what ordinary uffd (handle
> missing/unpopulated pages) and uffd-wp (handle write access to pages)
> can do with files instead. Because if something like that already
> exists, it would be precisely what I am talking about.

Every notification that uffd wants already exists as a notification to
the underlying filesystem.  Something like a uffdfs [1] would be able
to do everything that uffd does without adding extra crap all over the MM.

[1] acronyms are bad, mmmkay?