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

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Date:   Tue, 5 Oct 2021 12:14:59 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
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        Al Viro <viro@...iv.linux.org.uk>,
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        Kalesh Singh <kaleshsingh@...gle.com>,
        Peter Xu <peterx@...hat.com>, rppt@...nel.org,
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Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

On Tue, Oct 5, 2021 at 11:42 AM Pavel Machek <pavel@....cz> wrote:
>
> On Fri 2021-10-01 13:56:57, Suren Baghdasaryan wrote:
> > While forking a process with high number (64K) of named anonymous vmas the
> > overhead caused by strdup() is noticeable. Experiments with ARM64
> Android
>
> I still believe you should simply use numbers and do the
> numbers->strings mapping in userspace. We should not need to optimize
> strdups in kernel...

Here are complications with mapping numbers to strings in the userspace:
Approach 1: hardcode number->string in some header file and let all
tools use that mapping. The issue is that whenever that mapping
changes all the tools that are using it (including 3rd party ones)
have to be rebuilt. This is not really maintainable since we don't
control 3rd party tools and even for the ones we control, it will be a
maintenance issue figuring out which version of the tool used which
header file.
Approach 2: have a centralized facility (a process or a DB)
maintaining number->string mapping. This would require an additional
request to this facility whenever we want to make a number->string
conversion. Moreover, when we want to name a VMA, we would have to
register a new VMA name in that facility or check that one already
exists and get its ID. So each prctl() call to name a VMA will be
preceded by such a request (IPC call), maybe with some optimizations
to cache already known number->string pairs. This would be quite
expensive performance-wise. Additional issue with this approach is
that this mapping will have to be persistent to handle a case when the
facility crashes and has to be restored.

As I said before, it complicates userspace quite a bit. Is that a good
enough reason to store the names in the kernel and pay a little more
memory for that? IMHO yes, but I might be wrong.
Thanks,
Suren.

>
> Best regards,
>                                                                 Pavel
> --
> http://www.livejournal.com/~pavelmachek
>
> --
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