linux-kernel - Optimising USERNS mapping translation.

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Message-ID: <CAOviyaiRY5WA+kfSyaKKL7nJCeLOCgf5cCenbiRDHoO+M1NcAg@mail.gmail.com>
Date:	Tue, 15 Sep 2015 22:35:45 +1000
From:	Aleksa Sarai <cyphar@...har.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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: 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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