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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Oct 7, 2021 at 12:33 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 06.10.21 17:20, Suren Baghdasaryan wrote:
> > On Wed, Oct 6, 2021 at 8:08 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 06.10.21 17:01, Suren Baghdasaryan wrote:
> >>> On Wed, Oct 6, 2021 at 2:27 AM David Hildenbrand <david@redhat.com> 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).
>
> mmap() and prctl() already do take the mmap sem in write, so they are
> not the "most lightweight" operations so to say.
>
> We already to have two separate operations, first the mmap(), then the
> prctl(). IMHO you could defer the "naming" part to a later point in
> time, without creating too many issues, moving it out of the
> "hot/performance critical phase"
>
> Reading https://lwn.net/Articles/867818/, to me it feels like the use
> case could live with a little larger delay between the mmap popping up
> and a name getting assigned.

That might be doable if occasional inconsistency can be tolerated (we
can't guarantee that maps won't be read before the deferred work name
the vma). However I would prefer an efficient solution vs the one
which is inefficient but can be deferred.

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