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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 7 Oct 2021 09:04:09 -0700
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Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Rasmus Villemoes <linux@rasmusvillemoes.dk>,
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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?

> >
> > So in terms of syscall overhead, that would be open(..., O_CREAT |
> > O_CLOEXEC), fstat(), close() - or one could optimistically start by
>
> You could get to two if you used mkdir instead of open.
>
> > > YOUR_IDS_DIR can live on a tmpfs and you can even implement a policy on
> > > top of that (who can generate new ids, gurantee uniqness etc...).
> > >
> > > The above is certainly not for free of course but if you really need a
> > > system wide consistency when using names then you need some sort of
> > > central authority. How you implement that is not all that important
> > > but I do not think we want to handle that in the kernel.

Ideally it would be great if $YOUR_IDS_DIR/my_string_name entries
could be generated by the kernel in response to userspace calling
prctl(..., name) but I haven't looked into complexity of doing that,
so I would not propose that at this point.
Thanks for sharing the ideas!
Suren.

> >
> > IDK. If the whole thing could be put behind a CONFIG_ knob, with _zero_
> > overhead when not enabled (and I'm a bit worried about all the functions
> > that grow an extra argument that gets passed around), I don't mind the
> > string interface. But I don't really have a say either way.
>
> If this is ever useful outside of Android, eventually distros will
> have it enabled.
>
> Best regards,
>                                                                 Pavel
> --
> http://www.livejournal.com/~pavelmachek