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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting
Message-ID: <192438ab-a095-d441-6843-432fbbb8e38a@redhat.com>
Date:   Wed, 6 Oct 2021 17:07:55 +0200
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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

> 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?

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

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.

-- 
Thanks,

David / dhildenb