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

[Suren Baghdasaryan]

On Thu, Oct 7, 2021 at 9:40 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 07-10-21 09:04:09, Suren Baghdasaryan wrote:
> > On Thu, Oct 7, 2021 at 3:15 AM Pavel Machek <pavel@xxxxxx> 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?
Aside from the obvious inefficiency of requiring tools to parse
maps/smaps and convert ids to names it also creates a tool->system
dependency. Tools would have to know how the system interprets these
IDs and on the systems where the ID is an inode they would have to
know where the soflinks proposed by Rasmus reside and convert them. If
I'm a tool developer who wants the tool to work on multiple systems
this becomes quite messy.

>
> --
> Michal Hocko
> SUSE Labs