Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 26 Jan 2022 14:12:51 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Khalid Aziz <khalid.aziz@oracle.com>,
        akpm@linux-foundation.org, longpeng2@huawei.com, arnd@arndb.de,
        dave.hansen@linux.intel.com, rppt@kernel.org, surenb@google.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Peter Xu <peterx@redhat.com>
Subject: Re: [RFC PATCH 0/6] Add support for shared PTEs across processes
Message-ID: <YfFW43PT5hwPHnOz@casper.infradead.org>
References: <20220125114212.ks2qtncaahi6foan@box.shutemov.name>
 <Ye/5yUyEqO0ws0G5@casper.infradead.org>
 <20220125135917.ezi6itozrchsdcxg@box.shutemov.name>
 <YfAEqzTeBJSIOKcA@casper.infradead.org>
 <20220125185705.wf7p2l77vggipfry@box.shutemov.name>
 <YfBIpmxvc0+mFByf@casper.infradead.org>
 <YfDIYKygRHX4RIri@casper.infradead.org>
 <e164d7f4-406e-eed8-37d7-753f790b7560@redhat.com>
 <YfFO6VyMTrW1bWuu@casper.infradead.org>
 <2190b8e2-74f2-0e31-0a40-0401fbd9966e@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <2190b8e2-74f2-0e31-0a40-0401fbd9966e@redhat.com>
Precedence: bulk

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?