linux-kernel - Re: [patch 1/2] fs: mnt_want

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Date:	Tue, 10 Mar 2009 08:48:57 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <haveblue@...ibm.com>
Subject: Re: [patch 1/2] fs: mnt_want_write speedup

On Tue, Mar 10, 2009 at 03:37:18PM +0100, Nick Piggin wrote:
> costly, unfortunately). It results in about 900 bytes smaller code too. It
> does increase the size of a vfsmount, however.

Only on 64-bit SMP systems, and then only by four bytes.  And, best of
all, you can fix that if you care.  Look at this:

        /* --- cacheline 1 boundary (64 bytes) --- */
        struct list_head           mnt_child;            /*    64    16 */
        int                        mnt_flags;            /*    80     4 */

        /* XXX 4 bytes hole, try to pack */

        const char  *              mnt_devname;          /*    88     8 */
        struct list_head           mnt_list;             /*    96    16 */
        struct list_head           mnt_expire;           /*   112    16 */

So move mnt_flags to later in the struct (after the pointers), and move

> +#ifdef CONFIG_SMP
> +     int *mnt_writers;
> +#else
> +     int mnt_writers;
> +#endif

to be with the other pointers.  Bonus points for putting it between
        struct mnt_namespace *     mnt_ns;               /*   184     8 */
and
        int                        mnt_id;               /*   192     4 */

so that it doesn't become a new 4-byte hole for those incredibly common
64-bit uniprocessor builds.  *cough*.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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