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

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Date:   Wed, 26 Jan 2022 16:18:57 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        akpm@...ux-foundation.org, longpeng2@...wei.com, arnd@...db.de,
        dave.hansen@...ux.intel.com, david@...hat.com, surenb@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

On Wed, Jan 26, 2022 at 04:42:47PM +0300, Kirill A. Shutemov wrote:
> On Wed, Jan 26, 2022 at 04:04:48AM +0000, Matthew Wilcox wrote:
> > On Tue, Jan 25, 2022 at 06:59:50PM +0000, Matthew Wilcox wrote:
> > > On Tue, Jan 25, 2022 at 09:57:05PM +0300, Kirill A. Shutemov wrote:
>
> > So how about something like this ...
> > 
> > int mcreate(const char *name, int flags, mode_t mode);
> > 
> > creates a new mm_struct with a refcount of 2.  returns an fd (one
> > of the two refcounts) and creates a name for it (inside msharefs,
> > holds the other refcount).
> > 
> > You can then mmap() that fd to attach it to a chunk of your address
> > space.  Once attached, you can start to populate it by calling
> > mmap() and specifying an address inside the attached mm as the first
> > argument to mmap().
> 
> That is not what mmap() would normally do to an existing mapping. So it
> requires special treatment.
> 
> In general mmap() of a mm_struct scares me. I can't wrap my head around
> implications.
> 
> Like how does it work on fork()?
> 
> How accounting works? What happens on OOM?
> 
> What prevents creating loops, like mapping a mm_struct inside itself?
> 
> What mremap()/munmap() do to such mapping? Will it affect mapping of
> mm_struct or will it target mapping inside the mm_sturct?
> 
> Maybe it just didn't clicked for me, I donno.

My understanding was that the new mm_struct would be rather stripped and
will be used more as an abstraction for the shared page table, maybe I'm
totally wrong :)
 
> > Maybe mcreate() is just a library call, and it's really a thin wrapper
> > around open() that happens to know where msharefs is mounted.
> 
> -- 
>  Kirill A. Shutemov

-- 
Sincerely yours,
Mike.