Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

[David Hildenbrand]

On 08.10.21 09:25, Rasmus Villemoes wrote:
> On 07/10/2021 21.02, John Hubbard wrote:
> > On 10/7/21 11:50, Suren Baghdasaryan wrote:
> > ...
>
> > > > > The desire is for one of these two parties to be a human who can get
> > > > > the data and use it as is without additional conversions.
> > > > > /proc/$pid/maps could report FD numbers instead of pathnames, which
> > > > > could be converted to pathnames in userspace. However we do not do
> > > > > that because pathnames are more convenient for humans to identify a
> > > > > specific resource. Same logic applies here IMHO.
> > > >
> > > > Yes, please. It really seems like the folks that are interested in this
> > > > feature want strings. (I certainly do.) For those not interested in the
> > > > feature, it sounds like a CONFIG to keep it away would be sufficient.
> > > > Can we just move forward with that?
> > >
> > > Would love to if others are ok with this.
> >
> > If this doesn't get accepted, then another way forward would to continue
> > the ideas above to their logical conclusion, and create a new file system:
> > vma-fs.
>
> Or: Why can't the library/application that wants a VMA backed by memory
> to have some associated name not just
>
>    fd = open("/run/named-vmas/foobar#24", O_CLOEXEC|O_RDWR|O_EXCL|O_CREAT);
>    unlink("/run/named-vmas/foobar#24");
>    ftruncate(fd, ...);
>    mmap(fd);
>
> where /run/named-vmas is a tmpfs (probably with some per-user/per-app
> subdirs). That requires no changes in the kernel at all. Yes, it lacks
> the automatic cleanup of using real anon mmap in case there's a crash
> between open and unlink, but in an environment like Android that should
> be solvable.

I'm going to point out that we already do have names for memfds.

"The name supplied in name is used as a filename and will be displayed 
as  the  target  of  the corresponding  symbolic  link  in  the 
directory /proc/self/fd/." It's also displayed in /proc/self/maps.

So theoretically, without any kernel changes one might be able to 
achieve something as proposed in this patch via memfd_create().

There is one issue to be fixed:

MAP_PRIVATE on memfd results in a double memory consumption on any 
access via the mapping last time I checked (one page for pagecache, one 
page for private mapping).


--
Thanks,

David / dhildenb