From: Eric Biggers <ebiggers@xxxxxxxxxx> Document the proposed STATX_DIOALIGN support for statx() (https://lore.kernel.org/linux-fsdevel/20220616201506.124209-1-ebiggers@xxxxxxxxxx). Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> --- man2/open.2 | 43 ++++++++++++++++++++++++++++++++----------- man2/statx.2 | 32 +++++++++++++++++++++++++++++++- 2 files changed, 63 insertions(+), 12 deletions(-) diff --git a/man2/open.2 b/man2/open.2 index d1485999f..ef29847c3 100644 --- a/man2/open.2 +++ b/man2/open.2 @@ -1732,21 +1732,42 @@ 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 5.20, +.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 a8620be6f..fff0a63ec 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 @@ -244,8 +249,11 @@ STATX_SIZE Want stx_size STATX_BLOCKS Want stx_blocks STATX_BASIC_STATS [All of the above] STATX_BTIME Want stx_btime +STATX_ALL The same as STATX_BASIC_STATS | STATX_BTIME. + This is deprecated and should not be used. STATX_MNT_ID Want stx_mnt_id (since Linux 5.8) -STATX_ALL [All currently available fields] +STATX_DIOALIGN Want stx_dio_mem_align and stx_dio_offset_align + (since Linux 5.20; support varies by filesystem) .TE .in .PP @@ -406,6 +414,28 @@ 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 +.BR "" ( O_DIRECT ) +on this file. or 0 if direct I/O is not supported on this file. +.IP +.B STATX_DIOALIGN +.IR "" ( stx_dio_mem_align +and +.IR stx_dio_offset_align ) +is supported on block devices since Linux 5.20. +The support on regular files varies by filesystem; it is supported by ext4 and +f2fs since Linux 5.20. +.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). -- 2.36.1
