Not able to create a device mapper target node

Not able to create a device mapper target node

[monica g]

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Hi,

I wrote a basic device mapper driver which caches reads.

I gave dm msg prefix as below
 #define DM_MSG_PREFIX "cacheread".

When i try to create a device mapper node using the below command
echo "0 `blockdev --getsize /dev/sda2` cacheread /dev/sda2 0" | dmsetup
create cacheread

the system hangs.I am not getting any error messages.

When i try to create a device mapper node for linear target(this target is
already present with the device mapper code) using below command

echo "0 `blockdev --getsize /dev/sda2` linear /dev/sda2 0" | dmsetup create
linear
it succeeds and creates device node linear in /dev/mapper directory.

I am stuck with this issue and not able to proceed furthur.

Please let me know if you have any suggestions.

Any help is truely appreciated.

Thanks in advance.

Regards,
Monica

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Re: Not able to create a device mapper target node

[Zdenek Kabelac]

Dne 24.6.2013 09:19, monica g napsal(a):
> Hi,
>
> I wrote a basic device mapper driver which caches reads.
>
> I gave dm msg prefix as below
>   #define DM_MSG_PREFIX "cacheread".
>
> When i try to create a device mapper node using the below command
> echo "0 `blockdev --getsize /dev/sda2` cacheread /dev/sda2 0" | dmsetup create
> cacheread
>
> the system hangs.I am not getting any error messages.

Why not looking into a basic linear driver -

linux/drivers/md/dm-stripe.c

and start updating this driver for your needs.
Kernel device driver development is not easily debugable from userspace.

Also you might be interested to look at  dm-cache driver available from 3.9
kernel and which does much more advanced caching of reads as well as writes....

Regards

Zdenek


>
> When i try to create a device mapper node for linear target(this target is
> already present with the device mapper code) using below command
>
> echo "0 `blockdev --getsize /dev/sda2` linear /dev/sda2 0" | dmsetup create linear
> it succeeds and creates device node linear in /dev/mapper directory.
>
> I am stuck with this issue and not able to proceed furthur.
>
> Please let me know if you have any suggestions.
>
> Any help is truely appreciated.
>
> Thanks in advance.
>
> Regards,
> Monica
>
>
>
>
> --
> dm-devel mailing list
> dm-devel@redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel
>

Re: Not able to create a device mapper target node

[Mikulas Patocka]

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On Mon, 24 Jun 2013, monica g wrote:

> Hi,
> 
> I wrote a basic device mapper driver which caches reads.
> 
> I gave dm msg prefix as below
>  #define DM_MSG_PREFIX "cacheread".
> 
> When i try to create a device mapper node using the below command
> echo "0 `blockdev --getsize /dev/sda2` cacheread /dev/sda2 0" | dmsetup
> create cacheread
> 
> the system hangs.I am not getting any error messages.
> 
> When i try to create a device mapper node for linear target(this target is
> already present with the device mapper code) using below command
> 
> echo "0 `blockdev --getsize /dev/sda2` linear /dev/sda2 0" | dmsetup create
> linear
> it succeeds and creates device node linear in /dev/mapper directory.
> 
> I am stuck with this issue and not able to proceed furthur.
> 
> Please let me know if you have any suggestions.
> 
> Any help is truely appreciated.
> 
> Thanks in advance.
> 
> Regards,
> Monica

Hi

How to debug hangs:

If the system is still responsive, just the process that loads your device 
mapper target hangs, switch to console and use Alt-Sysrq-W (or run
echo w >/proc/sysrq-trigger;dmesg ). It should display a stacktrace, and 
from the stacktrace you can find where did it hang.

Otherwise (if you can't get the stacktrace or you don't find the reason 
for the hang from the stacktrace), find a function that is the most likely 
cause (in your case, it would be the constructor of your device mapper 
target, if you don't find the hang in the constructor, try other 
functions, such as a map routine) and put statements such as 
printk("step1\n") ... printk("step2\n") ... printk("step3\n") ... etc at 
various places in that function. Switch to console (don't do it in 
x-window), load your target and from the messages printed on the console, 
you can find out where did the hang happen - for example, if it prints 
"step1" and "step2" and doesn't print "step3", you know that the hang 
happened between step2 and step3.

Mikulas

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