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

[Andrew Morton]

On Wed, 6 Oct 2021 08:20:20 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:

> On Wed, Oct 6, 2021 at 8:08 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > On 06.10.21 17:01, Suren Baghdasaryan wrote:
> > > On Wed, Oct 6, 2021 at 2:27 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >>
> > >> On 06.10.21 10:27, Michal Hocko wrote:
> > >>> On Tue 05-10-21 23:57:36, John Hubbard wrote:
> > >>> [...]
> > >>>> 1) Yes, just leave the strings in the kernel, that's simple and
> > >>>> it works, and the alternatives don't really help your case nearly
> > >>>> enough.
> > >>>
> > >>> I do not have a strong opinion. Strings are easier to use but they
> > >>> are more involved and the necessity of kref approach just underlines
> > >>> that. There are going to be new allocations and that always can lead
> > >>> to surprising side effects.  These are small (80B at maximum) so the
> > >>> overall footpring shouldn't all that large by default but it can grow
> > >>> quite large with a very high max_map_count. There are workloads which
> > >>> really require the default to be set high (e.g. heavy mremap users). So
> > >>> if anything all those should be __GFP_ACCOUNT and memcg accounted.
> > >>>
> > >>> I do agree that numbers are just much more simpler from accounting,
> > >>> performance and implementation POV.
> > >>
> > >> +1
> > >>
> > >> I can understand that having a string can be quite beneficial e.g., when
> > >> dumping mmaps. If only user space knows the id <-> string mapping, that
> > >> can be quite tricky.
> > >>
> > >> However, I also do wonder if there would be a way to standardize/reserve
> > >> ids, such that a given id always corresponds to a specific user. If we
> > >> use an uint64_t for an id, there would be plenty room to reserve ids ...
> > >>
> > >> I'd really prefer if we can avoid using strings and instead using ids.
> > >
> > > I wish it was that simple and for some names like [anon:.bss] or
> > > [anon:dalvik-zygote space] reserving a unique id would work, however
> > > some names like [anon:dalvik-/system/framework/boot-core-icu4j.art]
> > > are generated dynamically at runtime and include package name.
> >
> > Valuable information
> 
> Yeah, I should have described it clearer the first time around.

If it gets this fancy then the 80 char limit is likely to become a
significant limitation and the choice should be explained & justified.

Why not 97?  1034?  Why not just strndup_user() and be done with it?

> > My question would be, if we really have to expose these strings to the
> > kernel, or if an id is sufficient. Sure, it would move complexity to
> > user space, but keeping complexity out of the kernel is usually a good idea.
> 
> My worry here is not the additional complexity on the userspace side
> but the performance hit we would have to endure due to these
> conversions.

Has the performance hit been quantified?

I've seen this many times down the ages.  Something which *could* be
done in userspace is instead done in the kernel because coordinating
userspace is Just So Damn Hard.  I guess the central problem is that
userspace isn't centrally coordinated.  I wish we were better at this.