Date: Tue, 23 Oct 2001 11:46:50 -0700 From: Luigi Rizzo <rizzo@aciri.org> To: Alfred Perlstein <bright@mu.org> Cc: net@FreeBSD.ORG Subject: Re: performance issues with M_PREPEND on clusters Message-ID: <20011023114650.C34494@iguana.aciri.org> In-Reply-To: <20011023132813.I15052@elvis.mu.org> References: <20011023110307.A34494@iguana.aciri.org> <20011023132813.I15052@elvis.mu.org>
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On Tue, Oct 23, 2001 at 01:28:13PM -0500, Alfred Perlstein wrote:
...
> > In the mbuf code, M_LEADINGSPACE always returns 0 when M_EXT is
> > set, instead of calling the second part of M_WRITABLE to check
> > whether there is a chance of writing into the cluster. This means
...
> I've seen this brought up before, this is not what you want to
> do otherwise you risk corrupting EXT data. The right thing to
> do is have the people allocating writable EXT bufs mark them
> as such. Other EXT type buffers such as sendfile bufs can not
> be written to no matter what the sharecount is.
Actually, the BSD code works the other way around:
* standard EXT bufs as returned by MCLGET _are_ writable if
m_ext.ext_free == NULL and their refcnt is 1 (they _must_ be,
otherwise we would have errors when m_free() decides to
dispose of a cluster, which is a form of writing),
and
* people allocating "other EXT type" implicitly mark the buffer
as not-writable by setting m_ext.ext_free to the proper routine.
This is certainly what the sendfile bufs that you mentioned do.
M_WRITABLE (used in the patch i proposed) does exactly the checks
that you do when you m_free() a cluster. If this is wrong, then
m_free() is broken!
Am i missing something else ?
cheers
luigi
#define M_WRITABLE(m) (!((m)->m_flags & M_EXT) || \
((m)->m_ext.ext_free == NULL && mclrefcnt[mtocl((m)->m_ext.ext_buf)] == 1))
----------------------------------+-----------------------------------------
Luigi RIZZO, luigi@iet.unipi.it . ACIRI/ICSI (on leave from Univ. di Pisa)
http://www.iet.unipi.it/~luigi/ . 1947 Center St, Berkeley CA 94704
Phone: (510) 666 2927
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