Message-ID: <20211008010425.atn6kzho7xmnet4a@revolver>
Date:   Fri, 8 Oct 2021 01:04:32 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
CC:     John Hubbard <jhubbard@...dia.com>,
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Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

* Suren Baghdasaryan <surenb@...gle.com> [211007 17:32]:
> On Thu, Oct 7, 2021 at 12:03 PM 'John Hubbard' via kernel-team
> <kernel-team@...roid.com> wrote:
> >
> > 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.
> 
> TBH, I would prefer to have the current simple solution protected with
> 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.

I think the current locking around VMAs makes this a very tricky, if not
impossible, path.  Watching a proc file which takes the mmap_lock() is
painful enough.  Considering how hard it has been to have this feature
added, I cannot see locking changes being accepted as a more feasible
approach nor can I see increased mmap_lock() contention from any feature
being desired.

I like the CONFIG option.  The patches are in good shape and have a
clever way around the (unlikely?) scalability issue that existed.

Thanks,
Liam

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