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

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Date:   Wed, 26 Jan 2022 15:30:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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 26.01.22 15:12, Matthew Wilcox wrote:
> 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.

I don't speak "filesystem" fluently, but I assume that could be an
overlay over other fs?

Peter is currently upstreaming uffd-wp for shmem. How could that look
like when doing it the fs-way?

-- 
Thanks,

David / dhildenb