Re: [RFC PATCH v2 2/2] init: boot to device-mapper targets without an initr*

From: Will Drewry
Date: Mon May 17 2010 - 14:13:57 EST

On Mon, May 17, 2010 at 11:47 AM, Randy Dunlap <randy.dunlap@xxxxxxxxxx> wrote:
> On Fri, 14 May 2010 20:41:41 -0500 Will Drewry wrote:
>> Add a dm= kernel parameter modeled after the md= parameter from
>> do_mounts_md.  It allows for device-mapper targets to be configured at
>> boot time for use early in the boot process (as the root device or
>> otherwise).
> dm=<blah>
> documentation needs to be added to Documentation/kernel-parameters.txt and/or
> Documentation/md.txt.

Absolutely -- I've appended a proposed version below which I'll include in the
next round. I'm not a lkml veteran, so if it makes sense to resend
the full patchset
again with it, I can. Otherwise, I'll wait for comments on the code too.

>> The format is dm="name uuid ro,table line 1,table line 2,...".  The
>> parser expects the comma to be safe to use as a newline substitute but,
>> otherwise, uses the normal separator of space.  Some attempt has been
>> made to make it forgiving of additional spaces (using skip_spaces()).
> This "space in the arg string" has been tested, right?

Works for me so far. I've tested this on a user mode linux instance and
a real x86 machine with the dm-linear target and a custom dm target
(which I'll be
mailing to dm-devel@ for feedback later this week).

> It seems a bit odd to me.  Most kernel parameter strings that I am familiar with
> use punctuation for separating parameter parts.

Yup - I agree. My first cut I used commas with an empty ,, delimiter as a new
line. Then I ran across this in the source:

76 /* You can use " around spaces, but can't escape ". */
77 /* Hyphens and underscores equivalent in parameter names. */
78 static char *next_arg(char *args, char **param, char **val)

I will need to cross-check all the known dm targets to make sure that
they don't expect quoted internal strings. In my first look, I didn't see any.

However, I'm perfectly content with either approach so if there's a preference,
I'll rework it to make it happen. This approach just maps more cleanly to the
existing experience of configuring a mapped device.


documentation: dm= parameter documentation

Add documentation for early mapped device creation without an initial rd.

Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx>
Documentation/device-mapper/boot.txt | 36 ++++++++++++++++++++++++++++++++++
Documentation/kernel-parameters.txt | 4 +++
2 files changed, 40 insertions(+), 0 deletions(-)
create mode 100644 Documentation/device-mapper/boot.txt

diff --git a/Documentation/device-mapper/boot.txt
new file mode 100644
index 0000000..00fb557
--- /dev/null
+++ b/Documentation/device-mapper/boot.txt
@@ -0,0 +1,36 @@
+Boot time creation of mapped devices
+You can boot directly to a dm device using the following kernel command
+dm="<name> <uuid> <ro>,table line 1,...,table line n"
+name = the name to associated with the device
+ after boot, udev, if used, will use that name to label
+ the device node.
+uuid = may be 'none' or the UUID desired for the device.
+ro = may be 0 or 1. If non-zero, the device and device table will be
+ marked read-only.
+Each table line may be as normal when using the dmsetup tool except for
+two variations:
+1. Any use of commas will be interpreted as a newline
+2. Quotation marks cannot be escaped and cannot be used without
+ terminating the dm= argument.
+Unless renamed by udev, the device node created will be dm-0 as the
+first minor number for the device-mapper is used during early creation.
+- Booting to a linear array made up of user-mode linux block devices:
+ dm="lroot none 0, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" \
+ root=/dev/dm-0
+Will boot to a rw dm-linear target of 8192 sectors split across two
+block devices identified by their major:minor numbers. After boot, udev
+will rename this target to /dev/mapper/lroot (depending on the rules).
+No uuid was assigned.
diff --git a/Documentation/kernel-parameters.txt
index 839b21b..9e22f8e 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -43,6 +43,7 @@ parameter is applicable:
AVR32 AVR32 architecture is enabled.
AX25 Appropriate AX.25 support is enabled.
BLACKFIN Blackfin architecture is enabled.
+ DM Device mapper support is enabled.
DRM Direct Rendering Management support is enabled.
EDD BIOS Enhanced Disk Drive Services (EDD) is enabled
EFI EFI Partitioning (GPT) is enabled
@@ -654,6 +655,9 @@ and is between 256 and 4096 characters. It is
defined in the file
Disable PIN 1 of APIC timer
Can be useful to work around chipset bugs.

+ dm= [DM] Allows early creation of a device-mapper device.
+ See Documentation/device-mapper/boot.txt.
dmasound= [HW,OSS] Sound subsystem buffers

dma_debug=off If the kernel is compiled with DMA_API_DEBUG support,
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at