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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Oct 7, 2021 at 12:03 PM 'John Hubbard' via kernel-team
<kernel-team@android.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.
>
> thanks,
> --
> John Hubbard
> NVIDIA
>
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