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Date:   Thu, 7 Oct 2021 19:31:12 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Pavel Machek <pavel@ucw.cz>,
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Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting
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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.

-- 
Michal Hocko
SUSE Labs