1. Library Design¶

1.1. Introduction ¶

iomap is a filesystem library for handling common file operations. The library has two layers:

A lower layer that provides an iterator over ranges of file offsets. This layer tries to obtain mappings of each file ranges to storage from the filesystem, but the storage information is not necessarily required.

An upper layer that acts upon the space mappings provided by the lower layer iterator.

The iteration can involve mappings of file’s logical offset ranges to physical extents, but the storage layer information is not necessarily required, e.g. for walking cached file information. The library exports various APIs for implementing file operations such as:

Pagecache reads and writes

Folio write faults to the pagecache

Writeback of dirty folios

Direct I/O reads and writes

fsdax I/O reads, writes, loads, and stores

FIEMAP

lseek SEEK_DATA and SEEK_HOLE

swapfile activation

This origins of this library is the file I/O path that XFS once used; it has now been extended to cover several other operations.

1.2. Who Should Read This?¶

The target audience for this document are filesystem, storage, and pagecache programmers and code reviewers.

If you are working on PCI, machine architectures, or device drivers, you are most likely in the wrong place.

1.3. How Is This Better?¶

Unlike the classic Linux I/O model which breaks file I/O into small units (generally memory pages or blocks) and looks up space mappings on the basis of that unit, the iomap model asks the filesystem for the largest space mappings that it can create for a given file operation and initiates operations on that basis. This strategy improves the filesystem’s visibility into the size of the operation being performed, which enables it to combat fragmentation with larger space allocations when possible. Larger space mappings improve runtime performance by amortizing the cost of mapping function calls into the filesystem across a larger amount of data.

At a high level, an iomap operation looks like this:

For each byte in the operation range...
1. Obtain a space mapping via ->iomap_begin
2. For each sub-unit of work...
  1. Revalidate the mapping and go back to (1) above, if necessary. So far only the pagecache operations need to do this.
  2. Do the work
3. Increment operation cursor
4. Release the mapping via ->iomap_end, if necessary

Each iomap operation will be covered in more detail below. This library was covered previously by an LWN article and a KernelNewbies page.

The goal of this document is to provide a brief discussion of the design and capabilities of iomap, followed by a more detailed catalog of the interfaces presented by iomap. If you change iomap, please update this design document.

1.4. File Range Iterator ¶

1.4.1. Definitions ¶

buffer head: Shattered remnants of the old buffer cache.

fsblock: The block size of a file, also known as i_blocksize.

i_rwsem: The VFS struct inode rwsemaphore. Processes hold this in shared mode to read file state and contents. Some filesystems may allow shared mode for writes. Processes often hold this in exclusive mode to change file state and contents.

invalidate_lock: The pagecache struct address_space rwsemaphore that protects against folio insertion and removal for filesystems that support punching out folios below EOF. Processes wishing to insert folios must hold this lock in shared mode to prevent removal, though concurrent insertion is allowed. Processes wishing to remove folios must hold this lock in exclusive mode to prevent insertions. Concurrent removals are not allowed.

dax_read_lock: The RCU read lock that dax takes to prevent a device pre-shutdown hook from returning before other threads have released resources.

filesystem mapping lock: This synchronization primitive is internal to the filesystem and must protect the file mapping data from updates while a mapping is being sampled. The filesystem author must determine how this coordination should happen; it does not need to be an actual lock.

iomap internal operation lock: This is a general term for synchronization primitives that iomap functions take while holding a mapping. A specific example would be taking the folio lock while reading or writing the pagecache.

pure overwrite: A write operation that does not require any metadata or zeroing operations to perform during either submission or completion. This implies that the filesystem must have already allocated space on disk as IOMAP_MAPPED and the filesystem must not place any constraints on IO alignment or size. The only constraints on I/O alignment are device level (minimum I/O size and alignment, typically sector size).

1.4.2. `struct iomap`¶

The filesystem communicates to the iomap iterator the mapping of byte ranges of a file to byte ranges of a storage device with the structure below:

struct iomap {
    u64                 addr;
    loff_t              offset;
    u64                 length;
    u16                 type;
    u16                 flags;
    struct block_device *bdev;
    struct dax_device   *dax_dev;
    void                *inline_data;
    void                *private;
    const struct iomap_folio_ops *folio_ops;
    u64                 validity_cookie;
};

The fields are as follows:

offset and length describe the range of file offsets, in bytes, covered by this mapping. These fields must always be set by the filesystem.

type describes the type of the space mapping:

IOMAP_HOLE: No storage has been allocated. This type must never be returned in response to an IOMAP_WRITE operation because writes must allocate and map space, and return the mapping. The addr field must be set to IOMAP_NULL_ADDR. iomap does not support writing (whether via pagecache or direct I/O) to a hole.

IOMAP_DELALLOC: A promise to allocate space at a later time (“delayed allocation”). If the filesystem returns IOMAP_F_NEW here and the write fails, the ->iomap_end function must delete the reservation. The addr field must be set to IOMAP_NULL_ADDR.

IOMAP_MAPPED: The file range maps to specific space on the storage device. The device is returned in bdev or dax_dev. The device address, in bytes, is returned via addr.

IOMAP_UNWRITTEN: The file range maps to specific space on the storage device, but the space has not yet been initialized. The device is returned in bdev or dax_dev. The device address, in bytes, is returned via addr. Reads from this type of mapping will return zeroes to the caller. For a write or writeback operation, the ioend should update the mapping to MAPPED. Refer to the sections about ioends for more details.

IOMAP_INLINE: The file range maps to the memory buffer specified by inline_data. For write operation, the ->iomap_end function presumably handles persisting the data. The addr field must be set to IOMAP_NULL_ADDR.

flags describe the status of the space mapping. These flags should be set by the filesystem in ->iomap_begin:

IOMAP_F_NEW: The space under the mapping is newly allocated. Areas that will not be written to must be zeroed. If a write fails and the mapping is a space reservation, the reservation must be deleted.

IOMAP_F_DIRTY: The inode will have uncommitted metadata needed to access any data written. fdatasync is required to commit these changes to persistent storage. This needs to take into account metadata changes that may be made at I/O completion, such as file size updates from direct I/O.

IOMAP_F_SHARED: The space under the mapping is shared. Copy on write is necessary to avoid corrupting other file data.

IOMAP_F_BUFFER_HEAD: This mapping requires the use of buffer heads for pagecache operations. Do not add more uses of this.

IOMAP_F_MERGED: Multiple contiguous block mappings were coalesced into this single mapping. This is only useful for FIEMAP.

IOMAP_F_XATTR: The mapping is for extended attribute data, not regular file data. This is only useful for FIEMAP.

IOMAP_F_PRIVATE: Starting with this value, the upper bits can be set by the filesystem for its own purposes.

These flags can be set by iomap itself during file operations. The filesystem should supply an ->iomap_end function if it needs to observe these flags:

IOMAP_F_SIZE_CHANGED: The file size has changed as a result of using this mapping.

IOMAP_F_STALE: The mapping was found to be stale. iomap will call ->iomap_end on this mapping and then ->iomap_begin to obtain a new mapping.

Currently, these flags are only set by pagecache operations.

addr describes the device address, in bytes.

bdev describes the block device for this mapping. This only needs to be set for mapped or unwritten operations.

dax_dev describes the DAX device for this mapping. This only needs to be set for mapped or unwritten operations, and only for a fsdax operation.

inline_data points to a memory buffer for I/O involving IOMAP_INLINE mappings. This value is ignored for all other mapping types.

private is a pointer to filesystem-private information. This value will be passed unchanged to ->iomap_end.

folio_ops will be covered in the section on pagecache operations.

validity_cookie is a magic freshness value set by the filesystem that should be used to detect stale mappings. For pagecache operations this is critical for correct operation because page faults can occur, which implies that filesystem locks should not be held between ->iomap_begin and ->iomap_end. Filesystems with completely static mappings need not set this value. Only pagecache operations revalidate mappings; see the section about iomap_valid for details.

1.4.3. `struct iomap_ops`¶

Every iomap function requires the filesystem to pass an operations structure to obtain a mapping and (optionally) to release the mapping:

struct iomap_ops {
    int (*iomap_begin)(struct inode *inode, loff_t pos, loff_t length,
                       unsigned flags, struct iomap *iomap,
                       struct iomap *srcmap);

    int (*iomap_end)(struct inode *inode, loff_t pos, loff_t length,
                     ssize_t written, unsigned flags,
                     struct iomap *iomap);
};

1.4.3.1. `->iomap_begin`¶

iomap operations call ->iomap_begin to obtain one file mapping for the range of bytes specified by pos and length for the file inode. This mapping should be returned through the iomap pointer. The mapping must cover at least the first byte of the supplied file range, but it does not need to cover the entire requested range.

Each iomap operation describes the requested operation through the flags argument. The exact value of flags will be documented in the operation-specific sections below. These flags can, at least in principle, apply generally to iomap operations:

IOMAP_DIRECT is set when the caller wishes to issue file I/O to block storage.

IOMAP_DAX is set when the caller wishes to issue file I/O to memory-like storage.

IOMAP_NOWAIT is set when the caller wishes to perform a best effort attempt to avoid any operation that would result in blocking the submitting task. This is similar in intent to O_NONBLOCK for network APIs - it is intended for asynchronous applications to keep doing other work instead of waiting for the specific unavailable filesystem resource to become available. Filesystems implementing IOMAP_NOWAIT semantics need to use trylock algorithms. They need to be able to satisfy the entire I/O request range with a single iomap mapping. They need to avoid reading or writing metadata synchronously. They need to avoid blocking memory allocations. They need to avoid waiting on transaction reservations to allow modifications to take place. They probably should not be allocating new space. And so on. If there is any doubt in the filesystem developer’s mind as to whether any specific IOMAP_NOWAIT operation may end up blocking, then they should return -EAGAIN as early as possible rather than start the operation and force the submitting task to block. IOMAP_NOWAIT is often set on behalf of IOCB_NOWAIT or RWF_NOWAIT.

If it is necessary to read existing file contents from a different device or address range on a device, the filesystem should return that information via srcmap. Only pagecache and fsdax operations support reading from one mapping and writing to another.

1.4.3.2. `->iomap_end`¶

After the operation completes, the ->iomap_end function, if present, is called to signal that iomap is finished with a mapping. Typically, implementations will use this function to tear down any context that were set up in ->iomap_begin. For example, a write might wish to commit the reservations for the bytes that were operated upon and unreserve any space that was not operated upon. written might be zero if no bytes were touched. flags will contain the same value passed to ->iomap_begin. iomap ops for reads are not likely to need to supply this function.

Both functions should return a negative errno code on error, or zero on success.

1.5. Preparing for File Operations ¶

iomap only handles mapping and I/O. Filesystems must still call out to the VFS to check input parameters and file state before initiating an I/O operation. It does not handle obtaining filesystem freeze protection, updating of timestamps, stripping privileges, or access control.

1.6. Locking Hierarchy ¶

iomap requires that filesystems supply their own locking model. There are three categories of synchronization primitives, as far as iomap is concerned:

The upper level primitive is provided by the filesystem to coordinate access to different iomap operations. The exact primitive is specific to the filesystem and operation, but is often a VFS inode, pagecache invalidation, or folio lock. For example, a filesystem might take i_rwsem before calling iomap_file_buffered_write and iomap_file_unshare to prevent these two file operations from clobbering each other. Pagecache writeback may lock a folio to prevent other threads from accessing the folio until writeback is underway.

The lower level primitive is taken by the filesystem in the ->iomap_begin and ->iomap_end functions to coordinate access to the file space mapping information. The fields of the iomap object should be filled out while holding this primitive. The upper level synchronization primitive, if any, remains held while acquiring the lower level synchronization primitive. For example, XFS takes ILOCK_EXCL and ext4 takes i_data_sem while sampling mappings. Filesystems with immutable mapping information may not require synchronization here.

The operation primitive is taken by an iomap operation to coordinate access to its own internal data structures. The upper level synchronization primitive, if any, remains held while acquiring this primitive. The lower level primitive is not held while acquiring this primitive. For example, pagecache write operations will obtain a file mapping, then grab and lock a folio to copy new contents. It may also lock an internal folio state object to update metadata.

The exact locking requirements are specific to the filesystem; for certain operations, some of these locks can be elided. All further mentions of locking are recommendations, not mandates. Each filesystem author must figure out the locking for themself.

1.7. Bugs and Limitations ¶

No support for fscrypt.

No support for compression.

No support for fsverity yet.

Strong assumptions that IO should work the way it does on XFS.

Does iomap actually work for non-regular file data?

Patches welcome!