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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 6 Oct 2021 19:46:57 -0700
Message-ID: <CAJuCfpGLQK5aVe5zQfdkP=K4NBZXPjtG=ycjk3E4D64CAvVPsg@mail.gmail.com>
Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
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On Wed, Oct 6, 2021 at 7:29 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 6 Oct 2021 08:20:20 -0700 Suren Baghdasaryan <surenb@google.com> 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.
>
> 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?

The original patch from 8 years ago used 256 as the limit but Rasmus
argued that the string content should be human-readable, so 80 chars
seems to be a reasonable limit (see:
https://lore.kernel.org/all/d8619a98-2380-ca96-001e-60fe9c6204a6@rasmusvillemoes.dk),
which makes sense to me. We should be able to handle the 80 char limit
by trimming it before calling prctl().

>
> > > 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'll try to get the data that was collected or at least an estimate. I
imagine collecting such data would require considerable userspace
redesign.

> 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.

It's not just hard, it's also inefficient. And for our usecase
performance is important.

>
>
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