How does the kernel chooses the 'vfat' module for fat32 partitions?

How does the kernel chooses the 'vfat' module for fat32 partitions?

[Sudheer Divakaran]

Hi List,

I have one doubt regarding the component which determines that 'vfat'
module is the correct module to be used for accessing fat32 file
systems and how does vfat.ko  became the ideal candidate for accessing
fat32 partition? For example, even though my /etc/fstab doesn't
contain any entry for my usbstick, which is having a fat32 partition,
when I issued the command 'mount /dev/sdb1 /mnt/usb' after inserting
the usbstick,  I could see the following events in udevadm monitor
--kernel --udev' s output.  Pls note the lines

"KERNEL[1304999552.510965] add      /module/fat (module)
KERNEL[1304999552.516633] add      /module/vfat (module)"

[Questions]

1. Who makes the decision that vfat.ko is to be used for fat32
partitions? Is it kernel/udev or some other component?

2. How does the kernel or the module loader determines that 'vfat' is
the correct module to be used for accessing fat32 partition? AFAIK,
there is nothing similar to 'MODULE_DEVICE_TABLE' in vfat.ko's
sources, which maps partition type 'c' (partition type id for W95
FAT32) to vfat.ko (CMIIW) as in pci/usb drivers.

[udevadm monitor output]

KERNEL[1304999552.510965] add      /module/fat (module)
KERNEL[1304999552.512169] add      /kernel/slab/fat_cache (slab)
UDEV  [1304999552.512212] add      /module/fat (module)
KERNEL[1304999552.512239] add      /kernel/slab/fat_inode_cache (slab)
UDEV  [1304999552.512263] add      /kernel/slab/fat_cache (slab)
UDEV  [1304999552.512289] add      /kernel/slab/fat_inode_cache (slab)
KERNEL[1304999552.516633] add      /module/vfat (module)
UDEV  [1304999552.516861] add      /module/vfat (module)
KERNEL[1304999552.542026] add      /module/nls_cp437 (module)
UDEV  [1304999552.542267] add      /module/nls_cp437 (module)
KERNEL[1304999552.545320] add      /module/nls_iso8859_1 (module)
UDEV  [1304999552.545566] add      /module/nls_iso8859_1 (module)
[more entries...]

TIA,
-- 
Thanks
Sudheer

How does the kernel chooses the 'vfat' module for fat32 partitions?

[Mulyadi Santosa]

hi Sudheer...

On Tue, May 10, 2011 at 11:48, Sudheer Divakaran
<inbox1.sudheer@gmail.com> wrote:
> Hi List,
>
> I have one doubt regarding the component which determines that 'vfat'
> module is the correct module to be used for accessing fat32 file
> systems and how does vfat.ko ?became the ideal candidate for accessing
> fat32 partition?

AFAIK, kernel simply iterates the filesystem module so far loaded (the
ones you see in /proc/filesystems). Whichever match the superblock or
main metadata of the target partition, that is the one which is gonna
used...

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

How does the kernel chooses the 'vfat' module for fat32 partitions?

[Sudheer Divakaran]

Hi Mulyadi,

On Tue, May 10, 2011 at 12:10 PM, Mulyadi Santosa
<mulyadi.santosa@gmail.com> wrote:
> hi Sudheer...
>
> On Tue, May 10, 2011 at 11:48, Sudheer Divakaran
> <inbox1.sudheer@gmail.com> wrote:
>> Hi List,
>>
>> I have one doubt regarding the component which determines that 'vfat'
>> module is the correct module to be used for accessing fat32 file
>> systems and how does vfat.ko ?became the ideal candidate for accessing
>> fat32 partition?
>
> AFAIK, kernel simply iterates the filesystem module so far loaded (the
> ones you see in /proc/filesystems). Whichever match the superblock or
> main metadata of the target partition, that is the one which is gonna
> used...
>

vfat module was not loaded before, it got loaded only after I issued
the mount command. We can see this info in the udev log. Moreover
there was no entry for the usbstick partition in /etc/fstab. So
someone kernel/userspace takes the decision that vfat.ko should be
used for managing fat32 partitions. I was trying to identify this
component.


-- 
Thanks
Sudheer

How does the kernel chooses the 'vfat' module for fat32 partitions?

[Sudheer Divakaran]

Hi,

>>
>
> vfat module was not loaded before, it got loaded only after I issued
> the mount command. We can see this info in the udev log. Moreover
> there was no entry for the usbstick partition in /etc/fstab. So
> someone kernel/userspace takes the decision that vfat.ko should be
> used for managing fat32 partitions. I was trying to identify this
> component.
>

'mount' itself was responsible for guessing the vfat driver. Running
mount with -v gave the following output

# mount -v /dev/sdb1 /mnt/usb
mount: you didn't specify a filesystem type for /dev/sdb1
       I will try type vfat
/dev/sdb1 on /mnt/usb type vfat (rw)


-- 
Thanks
Sudheer

How does the kernel chooses the 'vfat' module for fat32 partitions?

[Mulyadi Santosa]

Hi...

On Tue, May 10, 2011 at 14:47, Sudheer Divakaran
<inbox1.sudheer@gmail.com> wrote:
> 'mount' itself was responsible for guessing the vfat driver. Running
> mount with -v gave the following output


we both are partly correct, from "man mount":
" If no -t option is given, or if  the  auto  type  is  specified,
              mount  will try to guess the desired type.  Mount uses the blkid
              or volume_id library for guessing the filesystem type;  if  that
              does not turn up anything that looks familiar, mount will try to
              read the file /etc/filesystems, or,  if  that  does  not  exist,
              /proc/filesystems.   All  of  the  filesystem types listed there
              will be tried, except for those that are labeled "nodev"  (e.g.,
              devpts,  proc and nfs).  If /etc/filesystems ends in a line with
              a single * only, mount will read /proc/filesystems afterwards.
"

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com