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Optimising USERNS mapping translation.

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  Optimising USERNS mapping translation. Aleksa Sarai <cyphar@cyphar.com> - 2015-09-15 14:40 +0200
    Re: Optimising USERNS mapping translation. "Eric W. Biederman" <ebiederm@xmission.com> - 2015-09-15 16:10 +0200

#1225039 — Optimising USERNS mapping translation.

Hi,

I'm wondering if there's a reason that we do a linear search for the
mapping translations between kuid_t and uid_t. Surely something like a
radix trie would (potentially) allow for better storage and lookup? Is
the whole "an extent fits in a cache line" a good enough optimisation
that it would be pointless to use a better structure? Also, is there a
good reason why we have UID_GID_MAP_MAX_EXTENTS = 5 (other than the
one cache line thing)? Surely it's not more efficient to have to
create a bunch of nested namespaces in order to have more than 5
extents?

-- 
Aleksa Sarai (cyphar)
www.cyphar.com
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#1225141

On September 15, 2015 7:35:45 AM CDT, Aleksa Sarai <cyphar@cyphar.com> wrote:
>Hi,
>
>I'm wondering if there's a reason that we do a linear search for the
>mapping translations between kuid_t and uid_t. Surely something like a
>radix trie would (potentially) allow for better storage and lookup? Is
>the whole "an extent fits in a cache line" a good enough optimisation
>that it would be pointless to use a better structure? Also, is there a
>good reason why we have UID_GID_MAP_MAX_EXTENTS = 5 (other than the
>one cache line thing)? Surely it's not more efficient to have to
>create a bunch of nested namespaces in order to have more than 5
>extents?

Is this a homework question?  It sounds like one.

If you need more than about two extents you are managing your uids very badly.

As for the nested namespaces and 5 extents limit.  I will let you read the code and think about that one.

Eric

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