Device Mapper mechanism in the Linux system kernel (1) (3)

3. The user space command calls the table_load function through ioctl. This function builds the mapping table of the specified mapped device and the mapped target device based on the parameters passed from the user space. This function first builds the corresponding dm_table and dm_target structures, then calls the dm_table_add_target function in dm-table.c to initialize these structures according to the parameters passed in by the user, and calls the construction function ctr of the corresponding target type according to the target type specified by the parameters. Build the structure corresponding to the target device in the memory, and then update the B-tree maintained in dm_table based on the established dm_target structure. After the above process is completed, add the created dm_table to the hash_cell structure corresponding to the global hash table of the mapped device.

4. Finally, the do_resume function is called through ioctl to establish the binding relationship between the mapped device and the mapping table. In fact, the process is to assign the current dm_table structure pointer value to the corresponding map field of mapped_device through the dm_swap_table function, and then modify the mapped_device representation. The domain of the current state.

Through the above 4 main steps, the device mapper creates a mapped device logical block device in the kernel that can be provided to users.

IO flow

The essential function of Device mapper is to forward IO requests from the logical device mapped device to the corresponding target device based on the mapping relationship and the IO processing rules described by the target driver. The Device mapper handles all block read and write IO requests directed to the mapped device from the generic_make_request and submit_bio interfaces of the block-level IO subsystem in the kernel. IO requests are processed from top to bottom in the device mapper's device tree through request forwarding. When a bio request is forwarded down the mapped device in the device tree, one or more bio clones are created and sent to the lower target device. The same process is then repeated at each level of the device tree. As long as the device tree is large enough, this forwarding process can theoretically continue indefinitely. At a certain level in the device tree, after the target driver ends a bio request, it reports the event indicating the end of the bio request to its upper mapped device. This process is performed at each level until the event is finally uploaded to the root mapped device. So far, the device mapper ends the original bio request on the root mapped device and ends the entire IO request process.

When Bio forwards layer by layer in the device mapper's device tree, it is finally forwarded to one or more leaf target nodes and terminates. Because a bio request cannot span multiple target devices (that is, physical space segments), at each level, the device mapper clones one or more bios based on the target mapping information of the mapped device notified in advance by the user, and disassembles the bios. After being divided, it is forwarded to the corresponding target device. These cloned bios are first handed over to the corresponding target driver on the mapped device for processing. The IO requests are filtered and processed according to the IO processing rules defined in the target driver, and then submitted to the target device for completion. The above process is completed in the dm_request function in the dm.c file. Target driver can process these bios as follows:

1. Queue these bios inside the driver for later processing;

2. Redirect the bios to one or more target devices or to each target device. Different sectors;

３. Return error status to device mapper.

The IO request is processed layer by layer in the device tree shown in Figure 2 according to the process described above until the IO request is completed.

Summary

Device mapper provides a mapping architecture from logical devices to physical devices in the kernel. As long as the user formulates a mapping strategy in the user space and writes a target driver plug-in that handles specific IO requests according to his or her own needs, It is easy to implement a logical volume manager similar to LVM. Device mapper provides external interfaces in the form of ioctl. Users use the device mapper library in user space to send ioctl commands to the character device of device mapper to complete inward communication. It also provides a desired event notification mechanism through ioctl, allowing the target driver to transmit certain IO-related events to user space.

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