Re: [PATCH] Documentation: document the design of iomap and how to port

Darrick J. Wong
2024-06-09T15:55:07Z

On Sat, Jun 08, 2024 at 11:36:30PM -0700, Christoph Hellwig wrote:
> On Fri, Jun 07, 2024 at 05:17:07PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > This is the fourth attempt at documenting the design of iomap and how to
>
> The number of attempts should go out of the final commit version..
>
> > port filesystems to use it. Apologies for all the rst formatting, but
> > it's necessary to distinguish code from regular text.
>
> Maybe we should do this as a normal text file and not rst then?
>
> > +.. SPDX-License-Identifier: GPL-2.0
> > +.. _iomap:
> > +
> > +..
> > + 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.
>
> Should this be in the document and not a README in the directory?
>
> That being said starting every sentence on a new line makes the text
> really hard to read. To the point that I'll really need to go off
> and reformat it before making it beyond the first few paragraphs.
> I'll try to do that and will return to it later, sorry for just
> dropping these procedural notes for now.

HTML version here, text version below.
https://djwong.org/docs/iomap.html

--D

VFS iomap Design and Porting

Introduction

iomap is a filesystem library for handling various filesystem
operations that involves mapping of file's logical offset ranges
to physical extents. 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. The library provides various APIs for
implementing various file and pagecache operations, such as:

* Pagecache reads and writes

* Folio write faults to the pagecache

* Writeback of dirty folios

* Direct I/O reads and writes

* FIEMAP

* lseek SEEK_DATA and SEEK_HOLE

* swapfile activation

Who Should Read This?

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

But Why?

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 a mapping function call into the filesystem across a larger
amount of data.

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

1. For each byte in the operation range...

1. Obtain space mapping via ->iomap_begin

2. For each sub-unit of work...

1. Revalidate the mapping and go back to (1) above, if
necessary

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.

Data Structures and Algorithms

Definitions

* bufferhead: 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.

* invalidate_lock: The pagecache struct address_space
rwsemaphore that protects against folio removal.

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);
};

The ->iomap_begin function is called to obtain one mapping for the
range of bytes specified by pos and length for the file inode.

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, but these principles apply
generally:

* For a write operation, IOMAP_WRITE will be set. Filesystems
must not return IOMAP_HOLE mappings.

* For any other operation, IOMAP_WRITE will not be set.

* For any operation targetting direct access to storage
(fsdax), IOMAP_DAX will be set.

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.

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.

struct iomap

The filesystem returns the mappings via the following structure.
For documentation purposes, the structure has been reordered to
group fields that go together logically.

struct iomap {
loff_t offset;
u64 length;

u16 type;
u16 flags;

u64 addr;
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 information is useful for translating file operations into
action. The actions taken are specific to the target of the
operation, such as disk cache, physical storage devices, or
another part of the kernel.

* 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 will return zeroes to userspace. For a write or
writeback operation, the ioend should update the
mapping to MAPPED.

* 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 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.

XXX: Should fsdax revalidate as well?

Validation

NOTE: 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 updating of
timestamps, stripping privileges, or access control.

Locking Hierarchy

iomap requires that filesystems provide their own locking. There
are no locks within iomap itself, though in the course of an
operation iomap may take other locks (e.g. folio/dax locks) as
part of an I/O operation. Locking with iomap can be split into two
categories: above and below iomap.

The upper level of lock must coordinate the iomap operation with
other iomap operations. Generally, the filesystem must take
VFS/pagecache locks such as i_rwsem or invalidate_lock before
calling into iomap. The exact locking requirements are specific to
the type of operation.

The lower level of lock must coordinate access to the mapping
information. This lock is filesystem specific and should be held
during ->iomap_begin while sampling the mapping and validity
cookie.

The general locking hierarchy in iomap is:

* VFS or pagecache lock

* Internal filesystem specific mapping lock

* iomap operation-specific lock

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

iomap Operations

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

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 once had to implement themselves. This means that
the filesystem need not know the details of allocating, mapping,
managing uptodate and dirty state, or writeback of pagecache
folios. Unless the filesystem explicitly opts in to buffer heads,
they will not be used, which makes buffered I/O much more
efficient, and willy much happier.

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

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 such files, the mapping
must be revalidated after the folio lock has been taken so
that iomap can manage the folio correctly. The filesystem's
->iomap_begin function must sample a sequence counter into
struct iomap::validity_cookie at the same time that it
populates the mapping fields. It must then provide a
->iomap_valid function to compare the validity cookie
against the source counter and return whether or not the
mapping is still valid. If the mapping is not valid, the
mapping will be sampled again.

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

* IOCB_NOWAIT: Only proceed with the I/O if mapping data are
already in memory, we do not have to initiate other I/O, and
we acquire all filesystem locks without blocking. Neither
this flag nor its definition RWF_NOWAIT actually define what
this flag means, so this is the best the author could come
up with.

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.

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

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.

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.

mmap Write Faults

The iomap_page_mkwrite function handles a write fault to a folio
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.

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_file_buffered_write_punch_delalloc 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 should supply a callback punch will 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.

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
take i_rwsem and invalidate_lock in exclusive mode.

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 take i_rwsem and
invalidate_lock in exclusive mode.

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 take
i_rwsem and invalidate_lock in exclusive mode.

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:

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 fs block in each dirty folio, even if the
mapping returned is longer than one fs block. Do not return
IOMAP_INLINE mappings here; the ->iomap_end function must
deal with persisting written data. 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 is required.

* prepare_ioend: Enables filesystems to transform the
writeback ioend or perform any other prepatory work before
the writeback I/O is submitted. A filesystem can override
the ->bi_end_io function for its own purposes, such as
kicking the ioend completion to a workqueue if the bio is
completed in interrupt context. This function is optional.

* discard_folio: iomap calls this function after ->map_blocks
fails 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.

Writeback ioend Completion

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 override the struct iomap_end::bio::bi_end_io with its
own function. This function should call iomap_finish_ioends after
finishing its own work.

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.

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. An optional ops parameter can be passed to
change the behavior of direct I/O. The done_before parameter
should be set if writes have been initiated prior to the call. The
direction of the I/O is determined from the iocb passed in.

The flags argument can be any 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: Allocating blocks, zeroing partial
blocks, and extensions of the file size are not allowed. The
entire file range must to map to a single written or
unwritten extent. This flag exists to enable issuing
concurrent direct IOs with only the shared i_rwsem held when
the file I/O range is not aligned to the filesystem block
size. -EAGAIN will be returned if the operation cannot
proceed.

* 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.

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

* IOCB_NOWAIT: Only proceed with the I/O if mapping data are
already in memory, we do not have to initiate other I/O, and
we acquire all filesystem locks without blocking.

* 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.

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: Read if mapping data are already in memory.
Does not initiate other I/O or block on filesystem locks.

Callers commonly hold i_rwsem in shared mode.

Direct Writes

A direct I/O write initiates a write I/O to the storage device to
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: Write if mapping data are already in memory.
Does not initiate other I/O or block on filesystem locks.

* IOMAP_OVERWRITE_ONLY: Allocating blocks and zeroing partial
blocks is not allowed. The entire file range must to map to
a single written or unwritten extent. The file I/O range
must be aligned to the filesystem block size.

Callers commonly hold i_rwsem in shared or exclusive mode.

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.

DAX I/O

Storage devices that can be directly mapped as memory support a
new access mode known as "fsdax".

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: Read if mapping data are already in memory.
Does not initiate other I/O or block on filesystem locks.

Callers commonly hold i_rwsem in shared mode.

fsdax Writes

A fsdax write initiates a memcpy to the storage device to 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: Write if mapping data are already in memory.
Does not initiate other I/O or block on filesystem locks.

* IOMAP_OVERWRITE_ONLY: Allocating blocks and zeroing partial
blocks is not allowed. The entire file range must to map to
a single written or unwritten extent. The file I/O range
must be aligned to the filesystem block size.

Callers commonly hold i_rwsem in exclusive mode.

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.

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_DIO 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.

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.

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.

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.

Extent Map Reporting (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.

Block Map Reporting (FIBMAP)

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, but this is unclear.

Porting Guide

Why Convert to iomap?

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.

How to Convert to iomap?

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 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.

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!