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Date:   Fri, 8 Oct 2021 09:43:59 +0200
From:   David Hildenbrand <>
To:     Rasmus Villemoes <>,
        John Hubbard <>,
        Suren Baghdasaryan <>,
        Kees Cook <>
Cc:     Michal Hocko <>, Pavel Machek <>,
        Andrew Morton <>,
        Colin Cross <>,
        Sumit Semwal <>,
        Dave Hansen <>,
        Matthew Wilcox <>,
        "Kirill A . Shutemov" <>,
        Vlastimil Babka <>,
        Johannes Weiner <>,
        Jonathan Corbet <>,
        Al Viro <>,
        Randy Dunlap <>,
        Kalesh Singh <>,
        Peter Xu <>,,
        Peter Zijlstra <>,
        Catalin Marinas <>,,
        Chinwen Chang (張錦文) 
        Axel Rasmussen <>,
        Andrea Arcangeli <>,
        Jann Horn <>,,
        Yu Zhao <>, Will Deacon <>,,,
        Hugh Dickins <>,,
        Jason Gunthorpe <>, Roman Gushchin <>,
        Thomas Gleixner <>,,
        Chris Hyser <>,
        Peter Collingbourne <>,
        "Eric W. Biederman" <>,
        Jens Axboe <>,,
        Rolf Eike Beer <>,
        Cyrill Gorcunov <>,
        Muchun Song <>,
        Viresh Kumar <>,
        Thomas Cedeno <>,,, LKML <>,,,
        linux-mm <>,
        kernel-team <>
Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

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).


David / dhildenb

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