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

[Suren Baghdasaryan]

On Thu, Oct 7, 2021 at 9:37 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 07-10-21 08:45:21, Suren Baghdasaryan wrote:
> > On Thu, Oct 7, 2021 at 12:59 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Wed 06-10-21 08:01:56, 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.
> > > > Packages are constantly evolving, new ones are developed, names can
> > > > change, etc. So assigning a unique id for these names is not really
> > > > feasible.
> > >
> > > I still do not follow. If you need a globaly consistent naming then
> > > you need clear rules for that, no matter whether that is number or a
> > > file. How do you handle this with strings currently?
> >
> > Some names represent standard categories, some are unique. A simple
> > tool could calculate and report the total for each name, a more
> > advanced tool might recognize some standard names and process them
> > differently. From kernel's POV, it's just a name used by the userspace
> > to categorize anonymous memory areas.
>
> OK, so there is no real authority or any real naming convention. You
> just hope that applications will behave so that the consumer of those
> names can make proper calls. Correct?
>
> In that case the same applies to numbers and I do not see any strong
> argument for strings other than it is more pleasing to a human eye when
> reading the file. And that doesn't sound like a strong argument to make
> the kernel more complicated. Functionally both approaches are equal from
> a practical POV.

I don't think that's correct. Names like [anon:.bss],
[anon:dalvik-zygote space] and
[anon:dalvik-/system/framework/boot-core-icu4j.art] provide user with
actionable information about the use of that memory or the allocator
using it. Names like [anon:1], [anon:2] and [anon:3] do not convey any
valuable information for the user until they are converted into
descriptive names.

> --
> Michal Hocko
> SUSE Labs