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

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Date:   Thu, 7 Oct 2021 12:02:55 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Suren Baghdasaryan <surenb@...gle.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Michal Hocko <mhocko@...e.com>, Pavel Machek <pavel@....cz>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        David Hildenbrand <david@...hat.com>,
        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 10/7/21 11:50, Suren Baghdasaryan wrote:
...
>>>>>>>>>> I believe Pavel meant something as simple as
>>>>>>>>>> $ YOUR_FILE=$YOUR_IDS_DIR/my_string_name
>>>>>>>>>> $ touch $YOUR_FILE
>>>>>>>>>> $ stat -c %i $YOUR_FILE
>>>>>>>
>>>>>>> Ah, ok, now I understand the proposal. Thanks for the clarification!
>>>>>>> So, this would use filesystem as a directory for inode->name mappings.
>>>>>>> One rough edge for me is that the consumer would still need to parse
>>>>>>> /proc/$pid/maps and convert [anon:inode] into [anon:name] instead of
>>>>>>> just dumping the content for the user. Would it be acceptable if we
>>>>>>> require the ID provided by prctl() to always be a valid inode and
>>>>>>> show_map_vma() would do the inode-to-filename conversion when
>>>>>>> generating maps/smaps files? I know that inode->dentry is not
>>>>>>> one-to-one mapping but we can simply output the first dentry name.
>>>>>>> WDYT?
>>>>>>
>>>>>> No. You do not want to dictate any particular way of the mapping. The
>>>>>> above is just one way to do that without developing any actual mapping
>>>>>> yourself. You just use a filesystem for that. Kernel doesn't and
>>>>>> shouldn't understand the meaning of those numbers. It has no business in
>>>>>> that.
>>>>>>
>>>>>> In a way this would be pushing policy into the kernel.
>>>>>
>>>>> I can see your point. Any other ideas on how to prevent tools from
>>>>> doing this id-to-name conversion themselves?
>>>>
>>>> I really fail to understand why you really want to prevent them from that.
>>>> Really, the whole thing is just a cookie that kernel maintains for memory
>>>> mappings so that two parties can understand what the meaning of that
>>>> mapping is from a higher level. They both have to agree on the naming
>>>> but the kernel shouldn't dictate any specific convention because the
>>>> kernel _doesn't_ _care_. These things are not really anything actionable
>>>> for the kernel. It is just a metadata.
>>>
>>> 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.  Like debug-fs and other special file systems, similar policy and
motivation. Also protected by a CONFIG option.

Actually this seems at least as natural as the procfs approach, especially
given the nature of these strings, which feel more like dir+file names, than
simple strings.

thanks,
-- 
John Hubbard
NVIDIA