Date:   Thu, 7 Oct 2021 09:33:10 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>, John Hubbard <jhubbard@...dia.com>,
        Pavel Machek <pavel@....cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

On 06.10.21 17:20, Suren Baghdasaryan wrote:
> On Wed, Oct 6, 2021 at 8:08 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 06.10.21 17:01, Suren Baghdasaryan wrote:
>>> 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.
>>
>> 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.

-- 
Thanks,

David / dhildenb

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