From: Eric Biggers <ebiggers@xxxxxxxxxx> Document the STATX_DIOALIGN support for statx() (https://git.kernel.org/linus/725737e7c21d2d25). Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> --- v4: formatting tweaks, as suggested by Alejandro v3: updated mentions of Linux version, fixed some punctuation, and added a Reviewed-by v2: rebased onto man-pages master branch, mentioned xfs, and updated link to patchset man2/open.2 | 52 +++++++++++++++++++++++++++++++++++++++++----------- man2/statx.2 | 31 +++++++++++++++++++++++++++++++ 2 files changed, 72 insertions(+), 11 deletions(-) diff --git a/man2/open.2 b/man2/open.2 index 57beb324a..8e4a063b4 100644 --- a/man2/open.2 +++ b/man2/open.2 @@ -1732,21 +1732,51 @@ of user-space buffers and the file offset of I/Os. In Linux alignment restrictions vary by filesystem and kernel version and might be absent entirely. -However there is currently no filesystem\-independent -interface for an application to discover these restrictions for a given -file or filesystem. -Some filesystems provide their own interfaces -for doing so, for example the +The handling of misaligned +.B O_DIRECT +I/Os also varies; +they can either fail with +.B EINVAL +or fall back to buffered I/O. +.PP +Since Linux 6.1, +.B O_DIRECT +support and alignment restrictions for a file can be queried using +.BR statx (2), +using the +.B STATX_DIOALIGN +flag. +Support for +.B STATX_DIOALIGN +varies by filesystem; +see +.BR statx (2). +.PP +Some filesystems provide their own interfaces for querying +.B O_DIRECT +alignment restrictions, +for example the .B XFS_IOC_DIOINFO operation in .BR xfsctl (3). +.B STATX_DIOALIGN +should be used instead when it is available. .PP -Under Linux 2.4, transfer sizes, the alignment of the user buffer, -and the file offset must all be multiples of the logical block size -of the filesystem. -Since Linux 2.6.0, alignment to the logical block size of the -underlying storage (typically 512 bytes) suffices. -The logical block size can be determined using the +If none of the above is available, +then direct I/O support and alignment restrictions +can only be assumed from known characteristics of the filesystem, +the individual file, +the underlying storage device(s), +and the kernel version. +In Linux 2.4, +most block device based filesystems require that +the file offset and the length and memory address of all I/O segments +be multiples of the filesystem block size +(typically 4096 bytes). +In Linux 2.6.0, +this was relaxed to the logical block size of the block device +(typically 512 bytes). +A block device's logical block size can be determined using the .BR ioctl (2) .B BLKSSZGET operation or from the shell using the command: diff --git a/man2/statx.2 b/man2/statx.2 index 2a85be7c0..84c35bdf3 100644 --- a/man2/statx.2 +++ b/man2/statx.2 @@ -61,7 +61,12 @@ struct statx { containing the filesystem where the file resides */ __u32 stx_dev_major; /* Major ID */ __u32 stx_dev_minor; /* Minor ID */ + __u64 stx_mnt_id; /* Mount ID */ + + /* Direct I/O alignment restrictions */ + __u32 stx_dio_mem_align; + __u32 stx_dio_offset_align; }; .EE .in @@ -247,6 +252,8 @@ STATX_BTIME Want stx_btime STATX_ALL The same as STATX_BASIC_STATS | STATX_BTIME. It is deprecated and should not be used. STATX_MNT_ID Want stx_mnt_id (since Linux 5.8) +STATX_DIOALIGN Want stx_dio_mem_align and stx_dio_offset_align + (since Linux 6.1; support varies by filesystem) .TE .in .PP @@ -407,6 +414,30 @@ This is the same number reported by .BR name_to_handle_at (2) and corresponds to the number in the first field in one of the records in .IR /proc/self/mountinfo . +.TP +.I stx_dio_mem_align +The alignment (in bytes) required for user memory buffers for direct I/O +.RB ( O_DIRECT ) +on this file, +or 0 if direct I/O is not supported on this file. +.IP +.B STATX_DIOALIGN +.RI ( stx_dio_mem_align +and +.IR stx_dio_offset_align ) +is supported on block devices since Linux 6.1. +The support on regular files varies by filesystem; +it is supported by ext4, f2fs, and xfs since Linux 6.1. +.TP +.I stx_dio_offset_align +The alignment (in bytes) required for file offsets and I/O segment lengths +for direct I/O +.BR "" ( O_DIRECT ) +on this file, +or 0 if direct I/O is not supported on this file. +This will only be nonzero if +.I stx_dio_mem_align +is nonzero, and vice versa. .PP For further information on the above fields, see .BR inode (7). base-commit: ab47278f252262dd9bd90f3386ffd7d8700fa25a -- 2.37.3
