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

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Message-ID: <CAJuCfpEPBM6ehQXgzp=g4SqtY6iaC8wuZ-CRE81oR1VOq7m4CA@mail.gmail.com>
Date:   Wed, 6 Oct 2021 08:01:56 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>, John Hubbard <jhubbard@...dia.com>,
        Pavel Machek <pavel@....cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>,
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        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
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        Jonathan Corbet <corbet@....net>,
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Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

On Wed, Oct 6, 2021 at 2:27 AM David Hildenbrand <david@...hat.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.
Packages are constantly evolving, new ones are developed, names can
change, etc. So assigning a unique id for these names is not really
feasible.
That leaves us with the central facility option, which as I described
in my previous email would be prohibitive from performance POV (IPC
every time we have a new name or want to convert id to name).

I'm all for simplicity but the simple approach of using ids instead of
names unfortunately would not work for our usecases.

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