2. Supported File Operations¶

Below are a discussion of the high level file operations that iomap implements.

2.1. Buffered I/O ¶

Buffered I/O is the default file I/O path in Linux. File contents are cached in memory (“pagecache”) to satisfy reads and writes. Dirty cache will be written back to disk at some point that can be forced via fsync and variants.

iomap implements nearly all the folio and pagecache management that filesystems have to implement themselves under the legacy I/O model. This means that the filesystem need not know the details of allocating, mapping, managing uptodate and dirty state, or writeback of pagecache folios. Under the legacy I/O model, this was managed very inefficiently with linked lists of buffer heads instead of the per-folio bitmaps that iomap uses. Unless the filesystem explicitly opts in to buffer heads, they will not be used, which makes buffered I/O much more efficient, and the pagecache maintainer much happier.

2.1.1. `struct address_space_operations`¶

The following iomap functions can be referenced directly from the address space operations structure:

iomap_dirty_folio

iomap_release_folio

iomap_invalidate_folio

iomap_is_partially_uptodate

The following address space operations can be wrapped easily:

read_folio

readahead

writepages

bmap

swap_activate

2.1.2. `struct iomap_folio_ops`¶

The ->iomap_begin function for pagecache operations may set the struct iomap::folio_ops field to an ops structure to override default behaviors of iomap:

struct iomap_folio_ops {
    struct folio *(*get_folio)(struct iomap_iter *iter, loff_t pos,
                               unsigned len);
    void (*put_folio)(struct inode *inode, loff_t pos, unsigned copied,
                      struct folio *folio);
    bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
};

iomap calls these functions:

get_folio: Called to allocate and return an active reference to a locked folio prior to starting a write. If this function is not provided, iomap will call iomap_get_folio. This could be used to set up per-folio filesystem state for a write.

put_folio: Called to unlock and put a folio after a pagecache operation completes. If this function is not provided, iomap will folio_unlock and folio_put on its own. This could be used to commit per-folio filesystem state that was set up by ->get_folio.

iomap_valid: The filesystem may not hold locks between ->iomap_begin and ->iomap_end because pagecache operations can take folio locks, fault on userspace pages, initiate writeback for memory reclamation, or engage in other time-consuming actions. If a file’s space mapping data are mutable, it is possible that the mapping for a particular pagecache folio can change in the time it takes to allocate, install, and lock that folio.

For the pagecache, races can happen if writeback doesn’t take i_rwsem or invalidate_lock and updates mapping information. Races can also happen if the filesytem allows concurrent writes. For such files, the mapping must be revalidated after the folio lock has been taken so that iomap can manage the folio correctly.

fsdax does not need this revalidation because there’s no writeback and no support for unwritten extents.

Filesystems subject to this kind of race must provide a ->iomap_valid function to decide if the mapping is still valid. If the mapping is not valid, the mapping will be sampled again.

To support making the validity decision, the filesystem’s ->iomap_begin function may set struct iomap::validity_cookie at the same time that it populates the other iomap fields. A simple validation cookie implementation is a sequence counter. If the filesystem bumps the sequence counter every time it modifies the inode’s extent map, it can be placed in the struct iomap::validity_cookie during ->iomap_begin. If the value in the cookie is found to be different to the value the filesystem holds when the mapping is passed back to ->iomap_valid, then the iomap should considered stale and the validation failed.

These struct kiocb flags are significant for buffered I/O with iomap:

IOCB_NOWAIT: Turns on IOMAP_NOWAIT.

2.1.3. Internal per-Folio State ¶

If the fsblock size matches the size of a pagecache folio, it is assumed that all disk I/O operations will operate on the entire folio. The uptodate (memory contents are at least as new as what’s on disk) and dirty (memory contents are newer than what’s on disk) status of the folio are all that’s needed for this case.

If the fsblock size is less than the size of a pagecache folio, iomap tracks the per-fsblock uptodate and dirty state itself. This enables iomap to handle both “bs < ps” filesystems and large folios in the pagecache.

iomap internally tracks two state bits per fsblock:

uptodate: iomap will try to keep folios fully up to date. If there are read(ahead) errors, those fsblocks will not be marked uptodate. The folio itself will be marked uptodate when all fsblocks within the folio are uptodate.

dirty: iomap will set the per-block dirty state when programs write to the file. The folio itself will be marked dirty when any fsblock within the folio is dirty.

iomap also tracks the amount of read and write disk IOs that are in flight. This structure is much lighter weight than struct buffer_head because there is only one per folio, and the per-fsblock overhead is two bits vs. 104 bytes.

Filesystems wishing to turn on large folios in the pagecache should call mapping_set_large_folios when initializing the incore inode.

2.1.4. Buffered Readahead and Reads ¶

The iomap_readahead function initiates readahead to the pagecache. The iomap_read_folio function reads one folio’s worth of data into the pagecache. The flags argument to ->iomap_begin will be set to zero. The pagecache takes whatever locks it needs before calling the filesystem.

2.1.5. Buffered Writes ¶

The iomap_file_buffered_write function writes an iocb to the pagecache. IOMAP_WRITE or IOMAP_WRITE | IOMAP_NOWAIT will be passed as the flags argument to ->iomap_begin. Callers commonly take i_rwsem in either shared or exclusive mode before calling this function.

2.1.5.1. mmap Write Faults ¶

The iomap_page_mkwrite function handles a write fault to a folio in the pagecache. IOMAP_WRITE | IOMAP_FAULT will be passed as the flags argument to ->iomap_begin. Callers commonly take the mmap invalidate_lock in shared or exclusive mode before calling this function.

2.1.5.2. Buffered Write Failures ¶

After a short write to the pagecache, the areas not written will not become marked dirty. The filesystem must arrange to cancel such reservations because writeback will not consume the reservation. The iomap_write_delalloc_release can be called from a ->iomap_end function to find all the clean areas of the folios caching a fresh (IOMAP_F_NEW) delalloc mapping. It takes the invalidate_lock.

The filesystem must supply a function punch to be called for each file range in this state. This function must only remove delayed allocation reservations, in case another thread racing with the current thread writes successfully to the same region and triggers writeback to flush the dirty data out to disk.

2.1.5.3. Zeroing for File Operations ¶

Filesystems can call iomap_zero_range to perform zeroing of the pagecache for non-truncation file operations that are not aligned to the fsblock size. IOMAP_ZERO will be passed as the flags argument to ->iomap_begin. Callers typically hold i_rwsem and invalidate_lock in exclusive mode before calling this function.

2.1.5.4. Unsharing Reflinked File Data ¶

Filesystems can call iomap_file_unshare to force a file sharing storage with another file to preemptively copy the shared data to newly allocate storage. IOMAP_WRITE | IOMAP_UNSHARE will be passed as the flags argument to ->iomap_begin. Callers typically hold i_rwsem and invalidate_lock in exclusive mode before calling this function.

2.1.6. Truncation ¶

Filesystems can call iomap_truncate_page to zero the bytes in the pagecache from EOF to the end of the fsblock during a file truncation operation. truncate_setsize or truncate_pagecache will take care of everything after the EOF block. IOMAP_ZERO will be passed as the flags argument to ->iomap_begin. Callers typically hold i_rwsem and invalidate_lock in exclusive mode before calling this function.

2.1.7. Pagecache Writeback ¶

Filesystems can call iomap_writepages to respond to a request to write dirty pagecache folios to disk. The mapping and wbc parameters should be passed unchanged. The wpc pointer should be allocated by the filesystem and must be initialized to zero.

The pagecache will lock each folio before trying to schedule it for writeback. It does not lock i_rwsem or invalidate_lock.

The dirty bit will be cleared for all folios run through the ->map_blocks machinery described below even if the writeback fails. This is to prevent dirty folio clots when storage devices fail; an -EIO is recorded for userspace to collect via fsync.

The ops structure must be specified and is as follows:

2.1.7.1. `struct iomap_writeback_ops`¶

struct iomap_writeback_ops {
    int (*map_blocks)(struct iomap_writepage_ctx *wpc, struct inode *inode,
                      loff_t offset, unsigned len);
    int (*prepare_ioend)(struct iomap_ioend *ioend, int status);
    void (*discard_folio)(struct folio *folio, loff_t pos);
};

The fields are as follows:

map_blocks: Sets wpc->iomap to the space mapping of the file range (in bytes) given by offset and len. iomap calls this function for each dirty fs block in each dirty folio, though it will reuse mappings for runs of contiguous dirty fsblocks within a folio. Do not return IOMAP_INLINE mappings here; the ->iomap_end function must deal with persisting written data. Do not return IOMAP_DELALLOC mappings here; iomap currently requires mapping to allocated space. Filesystems can skip a potentially expensive mapping lookup if the mappings have not changed. This revalidation must be open-coded by the filesystem; it is unclear if iomap::validity_cookie can be reused for this purpose. This function must be supplied by the filesystem.

prepare_ioend: Enables filesystems to transform the writeback ioend or perform any other preparatory work before the writeback I/O is submitted. This might include pre-write space accounting updates, or installing a custom ->bi_end_io function for internal purposes, such as deferring the ioend completion to a workqueue to run metadata update transactions from process context. This function is optional.

discard_folio: iomap calls this function after ->map_blocks fails to schedule I/O for any part of a dirty folio. The function should throw away any reservations that may have been made for the write. The folio will be marked clean and an -EIO recorded in the pagecache. Filesystems can use this callback to remove delalloc reservations to avoid having delalloc reservations for clean pagecache. This function is optional.

2.1.7.2. Pagecache Writeback Completion ¶

To handle the bookkeeping that must happen after disk I/O for writeback completes, iomap creates chains of struct iomap_ioend objects that wrap the bio that is used to write pagecache data to disk. By default, iomap finishes writeback ioends by clearing the writeback bit on the folios attached to the ioend. If the write failed, it will also set the error bits on the folios and the address space. This can happen in interrupt or process context, depending on the storage device.

Filesystems that need to update internal bookkeeping (e.g. unwritten extent conversions) should provide a ->prepare_ioend function to set struct iomap_end::bio::bi_end_io to its own function. This function should call iomap_finish_ioends after finishing its own work (e.g. unwritten extent conversion).

Some filesystems may wish to amortize the cost of running metadata transactions for post-writeback updates by batching them. They may also require transactions to run from process context, which implies punting batches to a workqueue. iomap ioends contain a list_head to enable batching.

Given a batch of ioends, iomap has a few helpers to assist with amortization:

iomap_sort_ioends: Sort all the ioends in the list by file offset.

iomap_ioend_try_merge: Given an ioend that is not in any list and a separate list of sorted ioends, merge as many of the ioends from the head of the list into the given ioend. ioends can only be merged if the file range and storage addresses are contiguous; the unwritten and shared status are the same; and the write I/O outcome is the same. The merged ioends become their own list.

iomap_finish_ioends: Finish an ioend that possibly has other ioends linked to it.

2.2. Direct I/O ¶

In Linux, direct I/O is defined as file I/O that is issued directly to storage, bypassing the pagecache. The iomap_dio_rw function implements O_DIRECT (direct I/O) reads and writes for files.

ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
                     const struct iomap_ops *ops,
                     const struct iomap_dio_ops *dops,
                     unsigned int dio_flags, void *private,
                     size_t done_before);

The filesystem can provide the dops parameter if it needs to perform extra work before or after the I/O is issued to storage. The done_before parameter tells the how much of the request has already been transferred. It is used to continue a request asynchronously when part of the request has already been completed synchronously.

The done_before parameter should be set if writes for the iocb have been initiated prior to the call. The direction of the I/O is determined from the iocb passed in.

The dio_flags argument can be set to any combination of the following values:

IOMAP_DIO_FORCE_WAIT: Wait for the I/O to complete even if the kiocb is not synchronous.

IOMAP_DIO_OVERWRITE_ONLY: Perform a pure overwrite for this range or fail with -EAGAIN. This can be used by filesystems with complex unaligned I/O write paths to provide an optimised fast path for unaligned writes. If a pure overwrite can be performed, then serialisation against other I/Os to the same filesystem block(s) is unnecessary as there is no risk of stale data exposure or data loss. If a pure overwrite cannot be performed, then the filesystem can perform the serialisation steps needed to provide exclusive access to the unaligned I/O range so that it can perform allocation and sub-block zeroing safely. Filesystems can use this flag to try to reduce locking contention, but a lot of detailed checking is required to do it correctly.

IOMAP_DIO_PARTIAL: If a page fault occurs, return whatever progress has already been made. The caller may deal with the page fault and retry the operation. If the caller decides to retry the operation, it should pass the accumulated return values of all previous calls as the done_before parameter to the next call.

These struct kiocb flags are significant for direct I/O with iomap:

IOCB_NOWAIT: Turns on IOMAP_NOWAIT.

IOCB_SYNC: Ensure that the device has persisted data to disk before completing the call. In the case of pure overwrites, the I/O may be issued with FUA enabled.

IOCB_HIPRI: Poll for I/O completion instead of waiting for an interrupt. Only meaningful for asynchronous I/O, and only if the entire I/O can be issued as a single struct bio.

IOCB_DIO_CALLER_COMP: Try to run I/O completion from the caller’s process context. See linux/fs.h for more details.

Filesystems should call iomap_dio_rw from ->read_iter and ->write_iter, and set FMODE_CAN_ODIRECT in the ->open function for the file. They should not set ->direct_IO, which is deprecated.

If a filesystem wishes to perform its own work before direct I/O completion, it should call __iomap_dio_rw. If its return value is not an error pointer or a NULL pointer, the filesystem should pass the return value to iomap_dio_complete after finishing its internal work.

2.2.1. Return Values ¶

iomap_dio_rw can return one of the following:

A non-negative number of bytes transferred.

-ENOTBLK: Fall back to buffered I/O. iomap itself will return this value if it cannot invalidate the page cache before issuing the I/O to storage. The ->iomap_begin or ->iomap_end functions may also return this value.

-EIOCBQUEUED: The asynchronous direct I/O request has been queued and will be completed separately.

Any of the other negative error codes.

2.2.2. Direct Reads ¶

A direct I/O read initiates a read I/O from the storage device to the caller’s buffer. Dirty parts of the pagecache are flushed to storage before initiating the read io. The flags value for ->iomap_begin will be IOMAP_DIRECT with any combination of the following enhancements:

IOMAP_NOWAIT, as defined previously.

Callers commonly hold i_rwsem in shared mode before calling this function.

2.2.3. Direct Writes ¶

A direct I/O write initiates a write I/O to the storage device from the caller’s buffer. Dirty parts of the pagecache are flushed to storage before initiating the write io. The pagecache is invalidated both before and after the write io. The flags value for ->iomap_begin will be IOMAP_DIRECT | IOMAP_WRITE with any combination of the following enhancements:

IOMAP_NOWAIT, as defined previously.

IOMAP_OVERWRITE_ONLY: Allocating blocks and zeroing partial blocks is not allowed. The entire file range must map to a single written or unwritten extent. The file I/O range must be aligned to the filesystem block size if the mapping is unwritten and the filesystem cannot handle zeroing the unaligned regions without exposing stale contents.

Callers commonly hold i_rwsem in shared or exclusive mode before calling this function.

2.2.4. `struct iomap_dio_ops:`¶

struct iomap_dio_ops {
    void (*submit_io)(const struct iomap_iter *iter, struct bio *bio,
                      loff_t file_offset);
    int (*end_io)(struct kiocb *iocb, ssize_t size, int error,
                  unsigned flags);
    struct bio_set *bio_set;
};

The fields of this structure are as follows:

submit_io: iomap calls this function when it has constructed a struct bio object for the I/O requested, and wishes to submit it to the block device. If no function is provided, submit_bio will be called directly. Filesystems that would like to perform additional work before (e.g. data replication for btrfs) should implement this function.

end_io: This is called after the struct bio completes. This function should perform post-write conversions of unwritten extent mappings, handle write failures, etc. The flags argument may be set to a combination of the following:

IOMAP_DIO_UNWRITTEN: The mapping was unwritten, so the ioend should mark the extent as written.

IOMAP_DIO_COW: Writing to the space in the mapping required a copy on write operation, so the ioend should switch mappings.

bio_set: This allows the filesystem to provide a custom bio_set for allocating direct I/O bios. This enables filesystems to stash additional per-bio information for private use. If this field is NULL, generic struct bio objects will be used.

Filesystems that want to perform extra work after an I/O completion should set a custom ->bi_end_io function via ->submit_io. Afterwards, the custom endio function must call iomap_dio_bio_end_io to finish the direct I/O.

2.3. DAX I/O ¶

Some storage devices can be directly mapped as memory. These devices support a new access mode known as “fsdax” that allows loads and stores through the CPU and memory controller.

2.3.1. fsdax Reads ¶

A fsdax read performs a memcpy from storage device to the caller’s buffer. The flags value for ->iomap_begin will be IOMAP_DAX with any combination of the following enhancements:

IOMAP_NOWAIT, as defined previously.

Callers commonly hold i_rwsem in shared mode before calling this function.

2.3.2. fsdax Writes ¶

A fsdax write initiates a memcpy to the storage device from the caller’s buffer. The flags value for ->iomap_begin will be IOMAP_DAX | IOMAP_WRITE with any combination of the following enhancements:

IOMAP_NOWAIT, as defined previously.

IOMAP_OVERWRITE_ONLY: The caller requires a pure overwrite to be performed from this mapping. This requires the filesystem extent mapping to already exist as an IOMAP_MAPPED type and span the entire range of the write I/O request. If the filesystem cannot map this request in a way that allows the iomap infrastructure to perform a pure overwrite, it must fail the mapping operation with -EAGAIN.

Callers commonly hold i_rwsem in exclusive mode before calling this function.

2.3.2.1. fsdax mmap Faults ¶

The dax_iomap_fault function handles read and write faults to fsdax storage. For a read fault, IOMAP_DAX | IOMAP_FAULT will be passed as the flags argument to ->iomap_begin. For a write fault, IOMAP_DAX | IOMAP_FAULT | IOMAP_WRITE will be passed as the flags argument to ->iomap_begin.

Callers commonly hold the same locks as they do to call their iomap pagecache counterparts.

2.3.3. fsdax Truncation, fallocate, and Unsharing ¶

For fsdax files, the following functions are provided to replace their iomap pagecache I/O counterparts. The flags argument to ->iomap_begin are the same as the pagecache counterparts, with IOMAP_DAX added.

dax_file_unshare

dax_zero_range

dax_truncate_page

Callers commonly hold the same locks as they do to call their iomap pagecache counterparts.

2.3.4. fsdax Deduplication ¶

Filesystems implementing the FIDEDUPERANGE ioctl must call the dax_remap_file_range_prep function with their own iomap read ops.

2.4. Seeking Files ¶

iomap implements the two iterating whence modes of the llseek system call.

2.4.1. SEEK_DATA ¶

The iomap_seek_data function implements the SEEK_DATA “whence” value for llseek. IOMAP_REPORT will be passed as the flags argument to ->iomap_begin.

For unwritten mappings, the pagecache will be searched. Regions of the pagecache with a folio mapped and uptodate fsblocks within those folios will be reported as data areas.

Callers commonly hold i_rwsem in shared mode before calling this function.

2.4.2. SEEK_HOLE ¶

The iomap_seek_hole function implements the SEEK_HOLE “whence” value for llseek. IOMAP_REPORT will be passed as the flags argument to ->iomap_begin.

For unwritten mappings, the pagecache will be searched. Regions of the pagecache with no folio mapped, or a !uptodate fsblock within a folio will be reported as sparse hole areas.

Callers commonly hold i_rwsem in shared mode before calling this function.

2.5. Swap File Activation ¶

The iomap_swapfile_activate function finds all the base-page aligned regions in a file and sets them up as swap space. The file will be fsync()’d before activation. IOMAP_REPORT will be passed as the flags argument to ->iomap_begin. All mappings must be mapped or unwritten; cannot be dirty or shared, and cannot span multiple block devices. Callers must hold i_rwsem in exclusive mode; this is already provided by swapon.

2.6. File Space Mapping Reporting ¶

iomap implements two of the file space mapping system calls.

2.6.1. FS_IOC_FIEMAP ¶

The iomap_fiemap function exports file extent mappings to userspace in the format specified by the FS_IOC_FIEMAP ioctl. IOMAP_REPORT will be passed as the flags argument to ->iomap_begin. Callers commonly hold i_rwsem in shared mode before calling this function.

2.6.2. FIBMAP (deprecated)¶

iomap_bmap implements FIBMAP. The calling conventions are the same as for FIEMAP. This function is only provided to maintain compatibility for filesystems that implemented FIBMAP prior to conversion. This ioctl is deprecated; do not add a FIBMAP implementation to filesystems that do not have it. Callers should probably hold i_rwsem in shared mode before calling this function, but this is unclear.