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

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Date:   Fri, 8 Oct 2021 09:43:59 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        John Hubbard <jhubbard@...dia.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Michal Hocko <mhocko@...e.com>, Pavel Machek <pavel@....cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Peter Xu <peterx@...hat.com>, rppt@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        vincenzo.frascino@....com,
        Chinwen Chang (張錦文) 
        <chinwen.chang@...iatek.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jann Horn <jannh@...gle.com>, apopple@...dia.com,
        Yu Zhao <yuzhao@...gle.com>, Will Deacon <will@...nel.org>,
        fenghua.yu@...el.com, thunder.leizhen@...wei.com,
        Hugh Dickins <hughd@...gle.com>, feng.tang@...el.com,
        Jason Gunthorpe <jgg@...pe.ca>, Roman Gushchin <guro@...com>,
        Thomas Gleixner <tglx@...utronix.de>, krisman@...labora.com,
        Chris Hyser <chris.hyser@...cle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jens Axboe <axboe@...nel.dk>, legion@...nel.org,
        Rolf Eike Beer <eb@...ix.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Muchun Song <songmuchun@...edance.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Cedeno <thomascedeno@...gle.com>, sashal@...nel.org,
        cxfcosmos@...il.com, LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>,
        kernel-team <kernel-team@...roid.com>
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).


-- 
Thanks,

David / dhildenb