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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Oct 7, 2021 at 10:31 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 07-10-21 09:58:02, Suren Baghdasaryan wrote:
> > On Thu, Oct 7, 2021 at 9:40 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Thu 07-10-21 09:04:09, Suren Baghdasaryan wrote:
> > > > On Thu, Oct 7, 2021 at 3:15 AM Pavel Machek <pavel@ucw.cz> wrote:
> > > > >
> > > > > Hi!
> > > > >
> > > > > > >> Hmm, so the suggestion is to have some directory which contains files
> > > > > > >> representing IDs, each containing the string name of the associated
> > > > > > >> vma? Then let's say we are creating a new VMA and want to name it. We
> > > > > > >> would have to scan that directory, check all files and see if any of
> > > > > > >> them contain the name we want to reuse the same ID.
> > > > > > >
> > > > > > > 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.

>
> --
> Michal Hocko
> SUSE Labs