linux-kernel - Re: [PATCH] ext2/3 as ext4

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Date:	Tue, 01 Dec 2009 18:58:05 -0700
From:	Richard Holden <aciddeath@...il.com>
To:	Philip Wernersbach <philip.wernersbach@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext2/3 as ext4

Philip Wernersbach wrote:
> Hello Everyone,
>      Below is a small patch I wrote for kernel version 2.6.31.6. It
> causes ext2/3 filesystems to be mounted using the ext4 driver, in a
> way that is transparent to the userspace (and kernelspace for that
> matter, as long as the code uses the kernel's mounting API) and
> requires no change to the userspace (/kernelspace). This patch is
> useful because: 1) It allows minimalist kernel fanatics, like myself,
> to drop 2 drivers from their builds without any loss in functionality.
> 2) It allows the ext2/3 filesystems to take advantage of the
> performance/general improvements in the ext4 driver. 3) In the future,
> with this patch, the ext2/3 driver code could be dropped from the
> kernel without any loss in functionality. I have been using this patch
> for 2 months now with no flaws on my home Linux box, which uses ext3
> drives on a daily basis.

I will have to give this a try on few of my dev boxes and see how it
works out.

> Let me know if you use this patch, if enough people use it I will
> write a proper KCONFIG and submit it for inclusion into the kernel.
>
> So without further ado, here is the patch.
>
> Sincerely,
> Philip Wernersbach <philip.wernersbach@...il.com>
>
> --- linux-2.6.31.6.orig/fs/namespace.c  2009-11-28 22:30:07.178682500
-0500
> +++ linux-2.6.31.6/fs/namespace.c       2009-11-28 22:32:42.813462139
-0500
> @@ -6,6 +6,8 @@
>  *
>  * Based on code from fs/super.c, copyright Linus Torvalds and others.
>  * Heavily rewritten.
> + *
> + * Uses ext2/3 as ext4 patch by Philip Wernersbach (November 2009)
>  */
>
>  #include <linux/syscalls.h>
> @@ -1838,6 +1840,34 @@
>        return n;
>  }
>
> +/*
> + * compare_fs_names(char *fs1, char *fs2) - Part of the ext2/3 as
ext4 patch
> + *
> + * The following is a standard string compare function, made to compare
> + * filesystem names, it returns 0 if they equal, and 1 if they do not.
> + * It only handles strings up to 255 characters though, so be cautious.
> + */
> +static int compare_fs_names(char *fs1, char *fs2)
> +{
> +               int i;
> +
> +               if (fs1 == 0L || fs2 == 0L) return 1;
> +
> +               for (i=0;i<256;i++)
> +               {
> +                       if (fs1[i] != fs2[i])
> +                               return 1;
> +                       if (fs1[i] == 0 && fs2[i] == 0)
> +                               return 0;
> +                       if (fs1[i] == 0)
> +                               return 1;
> +                       if (fs2[i] == 0)
> +                               return 1;
> +               }
> +               return 0;
> +}
> +
> +

Why the open coded strcmp?

>  int copy_mount_options(const void __user * data, unsigned long *where)
>  {
>        int i;
> @@ -1891,6 +1921,22 @@
>        struct path path;
>        int retval = 0;
>        int mnt_flags = 0;
> +
> +       /*
> +        * This causes ext2 filesystems to be mounted with the ext4
driver.
> +        *
> +        * Part of the ext2/3 as ext4 patch.
> +        */
> +       if (compare_fs_names(type_page, "ext2") == 0)
> +               type_page[3] = '4';
> +
> +       /*
> +        * This causes ext3 filesystems to be mounted with the ext4
driver.
> +        *
> +        * Part of the ext2/3 as ext4 patch.
> +        */
> +       if (compare_fs_names(type_page, "ext3") == 0)
> +               type_page[3] = '4';

I would put these 2 conditionals into a single if ||, and consolidate
the type_page setting into 1 place, then you limit where you handle the
ext2/ext3 filesystems inside of ext4.

>
>        /* Discard magic */
>        if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
> --
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-Richard Holden
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