[PATCHSET v30.1 04/15] xfs: atomic file content exchanges

Darrick J. Wong
2024-03-27T01:47:21Z

Hi all,

This series creates a new XFS_IOC_EXCHANGE_RANGE ioctl to exchange
ranges of bytes between two files atomically, and later adds
XFS_IOC_START_COMMIT and XFS_IOC_COMMIT_RANGE ioctls to perform the
exchange only if the target file has not been changed since a given
sampling point.

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.

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 are the proposed manual pages:

IOCTL-XFS-EXCHANGE-RANGE(2System Calls ManuIOCTL-XFS-EXCHANGE-RANGE(2)

NAME
ioctl_xfs_exchange_range - exchange the contents of parts of
two files

SYNOPSIS
#include <sys/ioctl.h>
#include <xfs/xfs_fs_staging.h>

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

DESCRIPTION
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.

XFS_EXCHANGE_RANGE_TO_EOF
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-
file2_offset)).

XFS_EXCHANGE_RANGE_DSYNC
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
effect.

XFS_EXCHANGE_RANGE_FILE1_WRITTEN
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
unit.

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

RETURN VALUE
On error, -1 is returned, and errno is set to indicate the er‐
ror.

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

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.

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

EPERM file1_fd or file2_fd are immutable.

ETXTBSY
One of the files is a swap file.

EUCLEAN
The filesystem is corrupt.

EXDEV file1_fd and file2_fd are not on the same mounted
filesystem.

CONFORMING TO
This API is XFS-specific.

USE CASES
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,
.flags = XFS_EXCHANGE_RANGE_TO_EOF,
};

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
write.

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,
.flags = XFS_EXCHANGE_RANGE_FILE1_WRITTEN |
XFS_EXCHANGE_RANGE_FILE1_DSYNC,
};

ioctl(fd, XFS_IOC_EXCHANGE_RANGE, &args);

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

SEE ALSO
ioctl(2)

XFS 2024-02-10 IOCTL-XFS-EXCHANGE-RANGE(2)
IOCTL-XFS-COMMIT-RANGE(2) System Calls ManualIOCTL-XFS-COMMIT-RANGE(2)

NAME
ioctl_xfs_start_commit - prepare to exchange the contents of
two files ioctl_xfs_commit_range - conditionally exchange the
contents of parts of two files

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

int ioctl(int file2_fd, XFS_IOC_START_COMMIT, struct xfs_com‐
mit_range *arg);

int ioctl(int file2_fd, XFS_IOC_COMMIT_RANGE, struct xfs_com‐
mit_range *arg);

DESCRIPTION
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 if file2_fd passes cer‐
tain freshness criteria.

Before exchanging the contents, the program must call the
XFS_IOC_START_COMMIT ioctl to sample freshness data for
file2_fd. If the sampled metadata does not match the file
metadata at commit time, XFS_IOC_COMMIT_RANGE will return
EBUSY.

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_commit_range {
__s32 file1_fd;
__u32 pad;
__u64 file1_offset;
__u64 file2_offset;
__u64 length;
__u64 flags;
__u64 file2_freshness[5];
};

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.

The field file2_freshness is an opaque field whose contents are
determined by the kernel. These file attributes are used to
confirm that file2_fd has not changed by another thread since
the current thread began staging its own update.

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.

XFS_EXCHANGE_RANGE_TO_EOF
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-
file2_offset)).

XFS_EXCHANGE_RANGE_DSYNC
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
effect.

XFS_EXCHANGE_RANGE_FILE1_WRITTEN
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
unit.

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

RETURN VALUE
On error, -1 is returned, and errno is set to indicate the er‐
ror.

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

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.

EBUSY The file2 inode number and timestamps supplied do not
match file2_fd.

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.

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

EPERM file1_fd or file2_fd are immutable.

ETXTBSY
One of the files is a swap file.

EUCLEAN
The filesystem is corrupt.

EXDEV file1_fd and file2_fd are not on the same mounted
filesystem.

CONFORMING TO
This API is XFS-specific.

USE CASES
Several use cases are imagined for this system call. Coordina‐
tion between multiple threads is performed by the kernel.

The first is a filesystem defragmenter, which copies the con‐
tents of a file into another file and wishes to exchange the
space mappings of the two files, provided that the original
file has not changed.

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;
struct xfs_commit_range args = {
.flags = XFS_EXCHANGE_RANGE_TO_EOF,
};

/* gather file2's freshness information */
ioctl(fd, XFS_IOC_START_COMMIT, &args);
fstat(fd, &sb);

/* make a fresh copy of the file with terrible alignment to avoid reflink */
clone_file_range(fd, NULL, temp_fd, NULL, 1, 0);
clone_file_range(fd, NULL, temp_fd, NULL, sb.st_size - 1, 0);

/* commit the entire update */
args.file1_fd = temp_fd;
ret = ioctl(fd, XFS_IOC_COMMIT_RANGE, &args);
if (ret && errno == EBUSY)
printf("file changed while defrag was underway
");

The second is a data storage program that wants to commit non-
contiguous updates to a file atomically. This program cannot
coordinate updates to the file and therefore relies on the ker‐
nel to reject the COMMIT_RANGE command if the file has been up‐
dated by someone else. This can be done by creating a tempo‐
rary file, calling FICLONE(2) to share the contents, and stag‐
ing the updates into the temporary file. The FULL_FILES flag
is recommended 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);
struct xfs_commit_range args = {
.flags = XFS_EXCHANGE_RANGE_TO_EOF,
};

/* gather file2's freshness information */
ioctl(fd, XFS_IOC_START_COMMIT, &args);

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 */
args.file1_fd = temp_fd;
ret = ioctl(fd, XFS_IOC_COMMIT_RANGE, &args);
if (ret && errno == EBUSY)
printf("file changed before commit; will roll back
");

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

SEE ALSO
ioctl(2)

XFS 2024-02-18 IOCTL-XFS-COMMIT-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
updates.

If you're going to start using this code, I strongly recommend pulling
from my git trees, which are linked below.

This has been running on the djcloud for months with no problems. Enjoy!
Comments and questions are, as always, welcome.

--D

kernel git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=atomic-file-updates

xfsprogs git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=atomic-file-updates

fstests git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfstests-dev.git/log/?h=atomic-file-updates

xfsdocs git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-documentation.git/log/?h=atomic-file-updates
---
Commits in this patchset:
* vfs: export remap and write check helpers
* xfs: introduce new file range exchange ioctl
* xfs: create a log 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
* docs: update swapext -> exchmaps language
* xfs: introduce new file range commit ioctls
* 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 | 16
fs/xfs/libxfs/xfs_fs.h | 70 +
fs/xfs/libxfs/xfs_log_format.h | 64 +
fs/xfs/libxfs/xfs_log_recover.h | 2
fs/xfs/libxfs/xfs_sb.c | 3
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 | 599 ++++++++++
fs/xfs/xfs_exchmaps_item.h | 64 +
fs/xfs/xfs_exchrange.c | 882 ++++++++++++++
fs/xfs/xfs_exchrange.h | 50 +
fs/xfs/xfs_ioctl.c | 126 ++
fs/xfs/xfs_log_recover.c | 2
fs/xfs/xfs_mount.h | 5
fs/xfs/xfs_super.c | 19
fs/xfs/xfs_symlink.c | 49 -
fs/xfs/xfs_trace.c | 2
fs/xfs/xfs_trace.h | 382 ++++++
include/linux/fs.h | 1
30 files changed, 3855 insertions(+), 175 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