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

[Suren Baghdasaryan]

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.

>
> > Packages are constantly evolving, new ones are developed, names can
> > change, etc. So assigning a unique id for these names is not really
> > feasible.
>
> So, you'd actually want to generate/reserve an id for a given string at
> runtime, assign that id to the VMA, and have a way to match id <->
> string somehow?

If we go with ids then yes, that is what we would have to do.

> That reservation service could be inside the kernel or even (better?) in
> user space. The service could for example de-duplicates strings.

Yes but it would require an IPC call to that service potentially on
every mmap() when we want to name a mapped vma. This would be
prohibitive. Even on consumption side, instead of just dumping
/proc/$pid/maps we would have to parse the file and convert all
[anon:id] into [anon:name] with each conversion requiring an IPC call
(assuming no id->name pair caching on the client side).

> 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.

> --
> Thanks,
>
> David / dhildenb
>