[GIT PULL 03/16] xfs: atomic file content exchanges

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Hi Chandan,

Please pull this branch with changes for xfs for 6.10-rc1.

As usual, I did a test-merge with the main upstream branch as of a few
minutes ago, and didn't see any conflicts.  Please let me know if you
encounter any problems.


The following changes since commit 15f78aa3eb07645e7bef15a53b4ae1c757907d2c:

xfs: constify xfs_bmap_is_written_extent (2024-04-15 14:54:12 -0700)

are available in the Git repository at:

https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git tags/atomic-file-updates-6.10_2024-04-15

for you to fetch changes up to 0730e8d8ba1d1507f1d7fd719e1f835ce69961fe:

xfs: enable logged file mapping exchange feature (2024-04-15 14:54:26 -0700)

xfs: atomic file content exchanges [v30.3 03/16]

This series creates a new XFS_IOC_EXCHANGE_RANGE ioctl to exchange
ranges of bytes between two files atomically.

This new functionality enables data storage programs to stage and commit
file updates such that reader programs will see either the old contents
or the new contents in their entirety, with no chance of torn writes.  A
successful call completion guarantees that the new contents will be seen
even if the system fails.

The ability to exchange file fork mappings between files in this manner
is critical to supporting online filesystem repair, which is built upon
the strategy of constructing a clean copy of a damaged structure and
committing the new structure into the metadata file atomically.  The
ioctls exist to facilitate testing of the new functionality and to
enable future application program designs.

User programs will be able to update files atomically by opening an
O_TMPFILE, reflinking the source file to it, making whatever updates
they want to make, and exchange the relevant ranges of the temp file
with the original file.  If the updates are aligned with the file block
size, a new (since v2) flag provides for exchanging only the written
areas.  Note that application software must quiesce writes to the file
while it stages an atomic update.  This will be addressed by a
subsequent series.

This mechanism solves the clunkiness of two existing atomic file update
mechanisms: for O_TRUNC + rewrite, this eliminates the brief period
where other programs can see an empty file.  For create tempfile +
rename, the need to copy file attributes and extended attributes for
each file update is eliminated.

However, this method introduces its own awkwardness -- any program
initiating an exchange now needs to have a way to signal to other
programs that the file contents have changed.  For file access mediated
via read and write, fanotify or inotify are probably sufficient.  For
mmaped files, that may not be fast enough.

Here is the proposed manual page:


ioctl_xfs_exchange_range  -  exchange  the contents of parts of
two files

#include <sys/ioctl.h>
#include <xfs/xfs_fs.h>

int ioctl(int file2_fd, XFS_IOC_EXCHANGE_RANGE, struct  xfs_ex‐
change_range *arg);

Given  a  range  of bytes in a first file file1_fd and a second
range of bytes in a second file  file2_fd,  this  ioctl(2)  ex‐
changes the contents of the two ranges.

Exchanges  are  atomic  with  regards to concurrent file opera‐
tions.  Implementations must guarantee that readers see  either
the old contents or the new contents in their entirety, even if
the system fails.

The system call parameters are conveyed in  structures  of  the
following form:

struct xfs_exchange_range {
__s32    file1_fd;
__u32    pad;
__u64    file1_offset;
__u64    file2_offset;
__u64    length;
__u64    flags;

The field pad must be zero.

The  fields file1_fd, file1_offset, and length define the first
range of bytes to be exchanged.

The fields file2_fd, file2_offset, and length define the second
range of bytes to be exchanged.

Both  files must be from the same filesystem mount.  If the two
file descriptors represent the same file, the byte ranges  must
not  overlap.   Most  disk-based  filesystems  require that the
starts of both ranges must be aligned to the file  block  size.
If  this  is  the  case, the ends of the ranges must also be so
aligned unless the XFS_EXCHANGE_RANGE_TO_EOF flag is set.

The field flags control the behavior of the exchange operation.

Ignore the length parameter.  All bytes in  file1_fd
from  file1_offset to EOF are moved to file2_fd, and
file2's size is set to  (file2_offset+(file1_length-
file1_offset)).   Meanwhile, all bytes in file2 from
file2_offset to EOF are moved to file1  and  file1's
size    is   set   to   (file1_offset+(file2_length-

Ensure that all modified in-core data in  both  file
ranges  and  all  metadata updates pertaining to the
exchange operation are flushed to persistent storage
before  the  call  returns.  Opening either file de‐
scriptor with O_SYNC or O_DSYNC will have  the  same

Only  exchange sub-ranges of file1_fd that are known
to contain data  written  by  application  software.
Each  sub-range  may  be  expanded (both upwards and
downwards) to align with the file  allocation  unit.
For files on the data device, this is one filesystem
block.  For files on the realtime  device,  this  is
the realtime extent size.  This facility can be used
to implement fast atomic  scatter-gather  writes  of
any  complexity for software-defined storage targets
if all writes are aligned  to  the  file  allocation

Check  the parameters and the feasibility of the op‐
eration, but do not change anything.

On error, -1 is returned, and errno is set to indicate the  er‐

Error  codes can be one of, but are not limited to, the follow‐

EBADF  file1_fd is not open for reading and writing or is  open
for  append-only  writes;  or  file2_fd  is not open for
reading and writing or is open for append-only writes.

EINVAL The parameters are not correct for  these  files.   This
error  can  also appear if either file descriptor repre‐
sents a device, FIFO, or socket.  Disk filesystems  gen‐
erally  require  the  offset  and length arguments to be
aligned to the fundamental block sizes of both files.

EIO    An I/O error occurred.

EISDIR One of the files is a directory.

ENOMEM The kernel was unable to allocate sufficient  memory  to
perform the operation.

ENOSPC There  is  not  enough  free space in the filesystem ex‐
change the contents safely.

The filesystem does not support exchanging bytes between
the two files.

EPERM  file1_fd or file2_fd are immutable.

One of the files is a swap file.

The filesystem is corrupt.

EXDEV  file1_fd  and  file2_fd  are  not  on  the  same mounted

This API is XFS-specific.

Several use cases are imagined for this system  call.   In  all
cases, application software must coordinate updates to the file
because the exchange is performed unconditionally.

The first is a data storage program that wants to  commit  non-
contiguous  updates  to a file atomically and coordinates write
access to that file.  This can be done by creating a  temporary
file, calling FICLONE(2) to share the contents, and staging the
updates into the temporary file.  The FULL_FILES flag is recom‐
mended  for this purpose.  The temporary file can be deleted or
punched out afterwards.

An example program might look like this:

int fd = open("/some/file", O_RDWR);
int temp_fd = open("/some", O_TMPFILE | O_RDWR);

ioctl(temp_fd, FICLONE, fd);

/* append 1MB of records */
lseek(temp_fd, 0, SEEK_END);
write(temp_fd, data1, 1000000);

/* update record index */
pwrite(temp_fd, data1, 600, 98765);
pwrite(temp_fd, data2, 320, 54321);
pwrite(temp_fd, data2, 15, 0);

/* commit the entire update */
struct xfs_exchange_range args = {
.file1_fd = temp_fd,

ioctl(fd, XFS_IOC_EXCHANGE_RANGE, &args);

The second is a software-defined  storage  host  (e.g.  a  disk
jukebox)  which  implements an atomic scatter-gather write com‐
mand.  Provided the exported disk's logical block size  matches
the file's allocation unit size, this can be done by creating a
temporary file and writing the data at the appropriate offsets.
It  is  recommended that the temporary file be truncated to the
size of the regular file before any writes are  staged  to  the
temporary  file  to avoid issues with zeroing during EOF exten‐
sion.  Use this call with the FILE1_WRITTEN  flag  to  exchange
only  the  file  allocation  units involved in the emulated de‐
vice's write command.  The temporary file should  be  truncated
or  punched out completely before being reused to stage another

An example program might look like this:

int fd = open("/some/file", O_RDWR);
int temp_fd = open("/some", O_TMPFILE | O_RDWR);
struct stat sb;
int blksz;

fstat(fd, &sb);
blksz = sb.st_blksize;

/* land scatter gather writes between 100fsb and 500fsb */
pwrite(temp_fd, data1, blksz * 2, blksz * 100);
pwrite(temp_fd, data2, blksz * 20, blksz * 480);
pwrite(temp_fd, data3, blksz * 7, blksz * 257);

/* commit the entire update */
struct xfs_exchange_range args = {
.file1_fd = temp_fd,
.file1_offset = blksz * 100,
.file2_offset = blksz * 100,
.length       = blksz * 400,

ioctl(fd, XFS_IOC_EXCHANGE_RANGE, &args);

Some filesystems may limit the amount of data or the number  of
extents that can be exchanged in a single call.


XFS                           2024-02-10   IOCTL-XFS-EXCHANGE-RANGE(2)

The reference implementation in XFS creates a new log incompat feature
and log intent items to track high level progress of swapping ranges of
two files and finish interrupted work if the system goes down.  Sample
code can be found in the corresponding changes to xfs_io to exercise the
use case mentioned above.

Note that this function is /not/ the O_DIRECT atomic untorn file writes
concept that has also been floating around for years.  It is also not
the RWF_ATOMIC patchset that has been shared.  This RFC is constructed
entirely in software, which means that there are no limitations other
than the general filesystem limits.

As a side note, the original motivation behind the kernel functionality
is online repair of file-based metadata.  The atomic file content
exchange is implemented as an atomic exchange of file fork mappings,
which means that we can implement online reconstruction of extended
attributes and directories by building a new one in another inode and
exchanging the contents.

Subsequent patchsets adapt the online filesystem repair code to use
atomic file exchanges.  This enables repair functions to construct a
clean copy of a directory, xattr information, symbolic links, realtime
bitmaps, and realtime summary information in a temporary inode.  If this
completes successfully, the new contents can be committed atomically
into the inode being repaired.  This is essential to avoid making
corruption problems worse if the system goes down in the middle of
running repair.

For userspace, this series also includes the userspace pieces needed to
test the new functionality, and a sample implementation of atomic file

This has been running on the djcloud for months with no problems.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>

Darrick J. Wong (15):
vfs: export remap and write check helpers
xfs: introduce new file range exchange ioctl
xfs: create a incompat flag for atomic file mapping exchanges
xfs: introduce a file mapping exchange log intent item
xfs: create deferred log items for file mapping exchanges
xfs: bind together the front and back ends of the file range exchange code
xfs: add error injection to test file mapping exchange recovery
xfs: condense extended attributes after a mapping exchange operation
xfs: condense directories after a mapping exchange operation
xfs: condense symbolic links after a mapping exchange operation
xfs: make file range exchange support realtime files
xfs: support non-power-of-two rtextsize with exchange-range
xfs: capture inode generation numbers in the ondisk exchmaps log item
docs: update swapext -> exchmaps language
xfs: enable logged file mapping exchange feature

.../filesystems/xfs/xfs-online-fsck-design.rst     |  259 ++--
fs/read_write.c                                    |    1 +
fs/remap_range.c                                   |    4 +-
fs/xfs/Makefile                                    |    3 +
fs/xfs/libxfs/xfs_defer.c                          |    6 +
fs/xfs/libxfs/xfs_defer.h                          |    2 +-
fs/xfs/libxfs/xfs_errortag.h                       |    4 +-
fs/xfs/libxfs/xfs_exchmaps.c                       | 1237 ++++++++++++++++++++
fs/xfs/libxfs/xfs_exchmaps.h                       |  123 ++
fs/xfs/libxfs/xfs_format.h                         |   26 +-
fs/xfs/libxfs/xfs_fs.h                             |   42 +
fs/xfs/libxfs/xfs_log_format.h                     |   66 +-
fs/xfs/libxfs/xfs_log_recover.h                    |    4 +
fs/xfs/libxfs/xfs_sb.c                             |    5 +
fs/xfs/libxfs/xfs_symlink_remote.c                 |   47 +
fs/xfs/libxfs/xfs_symlink_remote.h                 |    1 +
fs/xfs/libxfs/xfs_trans_space.h                    |    4 +
fs/xfs/xfs_error.c                                 |    3 +
fs/xfs/xfs_exchmaps_item.c                         |  614 ++++++++++
fs/xfs/xfs_exchmaps_item.h                         |   64 +
fs/xfs/xfs_exchrange.c                             |  804 +++++++++++++
fs/xfs/xfs_exchrange.h                             |   38 +
fs/xfs/xfs_ioctl.c                                 |    4 +
fs/xfs/xfs_log_recover.c                           |   33 +
fs/xfs/xfs_mount.h                                 |    2 +
fs/xfs/xfs_super.c                                 |   23 +
fs/xfs/xfs_symlink.c                               |   49 +-
fs/xfs/xfs_trace.c                                 |    2 +
fs/xfs/xfs_trace.h                                 |  327 ++++++
include/linux/fs.h                                 |    1 +
30 files changed, 3613 insertions(+), 185 deletions(-)
create mode 100644 fs/xfs/libxfs/xfs_exchmaps.c
create mode 100644 fs/xfs/libxfs/xfs_exchmaps.h
create mode 100644 fs/xfs/xfs_exchmaps_item.c
create mode 100644 fs/xfs/xfs_exchmaps_item.h
create mode 100644 fs/xfs/xfs_exchrange.c
create mode 100644 fs/xfs/xfs_exchrange.h

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