when does the kernel request what modules (using kmod/modprobe)?



certain aliases, e.g. block-major-8 => sd_mod, seem to be hard-coded
into modprobe, but may be overridden in modules.conf (so, when
modules.conf defines an alias for block-major-8, sd_mod won't be
loaded anymore unless explicitly configured).

modprobe is called by the kernel (kernel/kmod.c).




- accessing a character device node causes the corresponding
  char-major-<num> to be requested.

- accessing a block device node (e.g. /dev/sda) causes the
  corresponding block-major-<num> to be requested.

- scsi_hostadapter ...

- ... (TODO)

-----

Now, you want to load the whole USB storage stack (usb-storage,
usb-uhci, usbcore, sd_mod, scsi_mod) on (e.g.) "mount -t vfat /dev/sda
/mnt/usbs":

# "hardware abstraction layer"
alias usb_host_controller usb-uhci

# applications
below usb-storage usb_host_controller

below block-major-8 usb-storage


You now have to call the mount command *twice* to make it work. On the
1st call, all modules are loaded:


$ mount -t vfat /dev/sda /mnt/usbs
mount: /dev/sda is not a valid block device
$ 

syslog:

Dec 25 03:11:25 tock kernel: kmod: requesting module: vfat
Dec 25 03:11:25 tock kernel: kmod: requesting module: block-major-8
Dec 25 03:11:25 tock kernel: SCSI subsystem driver Revision: 1.00
Dec 25 03:11:25 tock kernel: usb.c: registered new driver hub
Dec 25 03:11:25 tock kernel: usb-uhci.c: $Revision: 1.1 $ time 01:25:55 Dec 25 2004
Dec 25 03:11:25 tock kernel: usb-uhci.c: High bandwidth mode enabled
Dec 25 03:11:25 tock kernel: PCI: Found IRQ 9 for device 00:04.2
Dec 25 03:11:25 tock kernel: PCI: Sharing IRQ 9 with 00:0d.0
Dec 25 03:11:25 tock kernel: usb-uhci.c: USB UHCI at I/O 0xd400, IRQ 9
Dec 25 03:11:25 tock kernel: usb-uhci.c: Detected 2 ports
Dec 25 03:11:25 tock kernel: usb.c: new USB bus registered, assigned bus number 1
Dec 25 03:11:25 tock kernel: hub.c: USB hub found
Dec 25 03:11:25 tock kernel: hub.c: 2 ports detected
Dec 25 03:11:25 tock kernel: usb-uhci.c: v1.275:USB Universal Host Controller Interface driver
Dec 25 03:11:25 tock kernel: Initializing USB Mass Storage driver...
Dec 25 03:11:25 tock kernel: usb.c: registered new driver usb-storage
Dec 25 03:11:25 tock kernel: USB Mass Storage support registered.
Dec 25 03:11:25 tock kernel: kmod: requesting module: scsi_hostadapter
Dec 25 03:11:25 tock kernel: hub.c: new USB device 00:04.2-2, assigned address 2
[kurze Pause]
Dec 25 03:11:26 tock kernel: scsi0 : SCSI emulation for USB Mass Storage devices
Dec 25 03:11:26 tock kernel:   Vendor: Q-MAX     Model: USB Flash Drive   Rev: 1.11
Dec 25 03:11:26 tock kernel:   Type:   Direct-Access                      ANSI SCSI revision: 02
Dec 25 03:11:26 tock kernel: Attached scsi removable disk sda at scsi0, channel 0, id 0, lun 0
Dec 25 03:11:26 tock kernel: SCSI device sda: 516096 512-byte hdwr sectors (264 MB)
Dec 25 03:11:26 tock kernel: sda: Write Protect is off
Dec 25 03:11:26 tock kernel:  sda:

loaded modules:

Module                  Size  Used by    Not tainted
sd_mod                 10012   0  (autoclean) (unused)
usb-storage            21948   0  (autoclean) (unused)
usb-uhci               21316   0  (autoclean) (unused)
usbcore                34816   0  (autoclean) [usb-storage usb-uhci]
scsi_mod               83304   2  (autoclean) [sd_mod usb-storage]
vfat                    9308   0  (autoclean)
fat                    29304   0  (autoclean) [vfat]



But only on the 2nd call the partition is actually mounted:

$ mount -t vfat /dev/sda /mnt/usbs
$

syslog:

Dec 25 03:12:28 tock kernel: kmod: requesting module: nls_cp437
Dec 25 03:12:29 tock kernel: kmod: requesting module: nls_iso8859-1

This is apparently caused by the asynchronous nature of kmod.c's
operation...


(the "kmod: requesting module: <bla>" outputs are caused by [1]).


[1]

patch to log each invocation of komd/request_module (in the hope that
we'll learn something more about when what modules are requested)

--- linux-2.4.21/kernel/kmod.c.orig     Sat Aug  3 02:39:46 2002
+++ linux-2.4.21/kernel/kmod.c  Sat Dec 25 01:07:11 2004
@@ -156,6 +156,9 @@
        char *argv[] = { modprobe_path, "-s", "-k", "--", (char*)module_name, NULL };
        int ret;
 
+        printk(KERN_INFO
+               "kmod: requesting module: %s\n", (char*) module_name);
+
        ret = exec_usermodehelper(modprobe_path, argv, envp);
        if (ret) {
                printk(KERN_ERR