3. Porting Your Filesystem

3.1. Why Convert?

There are several reasons to convert a filesystem to iomap:

1. The classic Linux I/O path is not terribly efficient. Pagecache operations lock a single base page at a time and then call into the filesystem to return a mapping for only that page. Direct I/O operations build I/O requests a single file block at a time. This worked well enough for direct/indirect-mapped filesystems such as ext2, but is very inefficient for extent-based filesystems such as XFS.

2. Large folios are only supported via iomap; there are no plans to convert the old buffer_head path to use them.

3. Direct access to storage on memory-like devices (fsdax) is only supported via iomap.

4. Lower maintenance overhead for individual filesystem maintainers. iomap handles common pagecache related operations itself, such as allocating, instantiating, locking, and unlocking of folios. No ->write_begin(), ->write_end() or direct_IO address_space_operations are required to be implemented by filesystem using iomap.

3.2. How Do I Convert a Filesystem?

First, add #include <linux/iomap.h> from your source code and add select FS_IOMAP to your filesystem’s Kconfig option. Build the kernel, run fstests with the -g all option across a wide variety of your filesystem’s supported configurations to build a baseline of which tests pass and which ones fail.

The recommended approach is first to implement ->iomap_begin (and ->iomap_end if necessary) to allow iomap to obtain a read-only mapping of a file range. In most cases, this is a relatively trivial conversion of the existing get_block() function for read-only mappings. FS_IOC_FIEMAP is a good first target because it is trivial to implement support for it and then to determine that the extent map iteration is correct from userspace. If FIEMAP is returning the correct information, it’s a good sign that other read-only mapping operations will do the right thing.

Next, modify the filesystem’s get_block(create = false) implementation to use the new ->iomap_begin implementation to map file space for selected read operations. Hide behind a debugging knob the ability to switch on the iomap mapping functions for selected call paths. It is necessary to write some code to fill out the bufferhead-based mapping information from the iomap structure, but the new functions can be tested without needing to implement any iomap APIs.

Once the read-only functions are working like this, convert each high level file operation one by one to use iomap native APIs instead of going through get_block(). Done one at a time, regressions should be self evident. You do have a regression test baseline for fstests, right? It is suggested to convert swap file activation, SEEK_DATA, and SEEK_HOLE before tackling the I/O paths. A likely complexity at this point will be converting the buffered read I/O path because of bufferheads. The buffered read I/O paths doesn’t need to be converted yet, though the direct I/O read path should be converted in this phase.

At this point, you should look over your ->iomap_begin function. If it switches between large blocks of code based on dispatching of the flags argument, you should consider breaking it up into per-operation iomap ops with smaller, more cohesive functions. XFS is a good example of this.

The next thing to do is implement get_blocks(create == true) functionality in the ->iomap_begin/->iomap_end methods. It is strongly recommended to create separate mapping functions and iomap ops for write operations. Then convert the direct I/O write path to iomap, and start running fsx w/ DIO enabled in earnest on filesystem. This will flush out lots of data integrity corner case bugs that the new write mapping implementation introduces.

Now, convert any remaining file operations to call the iomap functions. This will get the entire filesystem using the new mapping functions, and they should largely be debugged and working correctly after this step.

Most likely at this point, the buffered read and write paths will still need to be converted. The mapping functions should all work correctly, so all that needs to be done is rewriting all the code that interfaces with bufferheads to interface with iomap and folios. It is much easier first to get regular file I/O (without any fancy features like fscrypt, fsverity, compression, or data=journaling) converted to use iomap. Some of those fancy features (fscrypt and compression) aren’t implemented yet in iomap. For unjournalled filesystems that use the pagecache for symbolic links and directories, you might also try converting their handling to iomap.

The rest is left as an exercise for the reader, as it will be different for every filesystem. If you encounter problems, email the people and lists in get_maintainers.pl for help.