Documentation/filesystems/iomap/operations.rst
.. SPDX-License-Identifier: GPL-2.0 .. _iomap_operations:
.. Dumb style notes to maintain the author's sanity: Please try to start sentences on separate lines so that sentence changes don't bleed colors in diff. Heading decorations are documented in sphinx.rst.
.. contents:: Table of Contents :local:
Below are a discussion of the high level file operations that iomap implements.
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.
struct address_space_operationsThe following iomap functions can be referenced directly from the address space operations structure:
iomap_dirty_folioiomap_release_folioiomap_invalidate_folioiomap_is_partially_uptodateThe following address space operations can be wrapped easily:
read_folioreadaheadwritepagesbmapswap_activatestruct iomap_folio_opsThe ->iomap_begin function for pagecache operations may set the
struct iomap::folio_ops field to an ops structure to override
default behaviors of iomap:
.. code-block:: c
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 <https://lore.kernel.org/all/[email protected]/>_
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 <https://lore.kernel.org/all/[email protected]/>_
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 <https://lore.kernel.org/all/[email protected]/>_
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.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 <https://lore.kernel.org/all/[email protected]/>_
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.
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.
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.
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.
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 <https://lore.kernel.org/all/[email protected]/>_
such reservations <https://lore.kernel.org/linux-xfs/[email protected]/>_
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.
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.
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.
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.
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:
struct iomap_writeback_ops
.. code-block:: c
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
<https://lore.kernel.org/all/[email protected]/>`_
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
<https://lore.kernel.org/all/[email protected]/>`_
delalloc reservations to avoid having delalloc reservations for
clean pagecache.
This function is optional.
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 <https://lore.kernel.org/all/[email protected]/>_
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.
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.
.. code-block:: c
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 <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c03098d4b9ad76bca2966a8769dcfe59f7f85103>_
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 <https://lore.kernel.org/linux-ext4/[email protected]/>_
is required to do it correctly <https://lore.kernel.org/linux-ext4/[email protected]/>_.
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.
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.
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.
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.
struct iomap_dio_ops:.. code-block:: c
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 <https://lore.kernel.org/all/[email protected]/>_
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.
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.
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.
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.
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.
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.
fsdax Deduplication
-------------------
Filesystems implementing the ``FIDEDUPERANGE`` ioctl must call the
``dax_remap_file_range_prep`` function with their own iomap read ops.
Seeking Files
=============
iomap implements the two iterating whence modes of the ``llseek`` system
call.
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.
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.
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``.
File Space Mapping Reporting
============================
iomap implements two of the file space mapping system calls.
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.
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.