linux-kernel - Re: Optimising USERNS mapping translation.

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Message-ID: <8EE75052-6B7C-41D3-8DD0-4E85DF621C4C@xmission.com>
Date:	Tue, 15 Sep 2015 09:01:52 -0500
From:	"Eric W. Biederman" <ebiederm@...ssion.com>
To:	Aleksa Sarai <cyphar@...har.com>
CC:	Andy Lutomirski <luto@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Lameter <cl@...ux.com>,
	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: Optimising USERNS mapping translation.



On September 15, 2015 7:35:45 AM CDT, Aleksa Sarai <cyphar@...har.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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