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Date:   Fri, 22 Jul 2022 00:42:28 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-man@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-api@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Keith Busch <kbusch@...nel.org>
Subject: [man-pages RFC PATCH v2] statx.2, open.2: document STATX_DIOALIGN

From: Eric Biggers <ebiggers@...gle.com>

Document the proposed STATX_DIOALIGN support for statx()
(https://lore.kernel.org/linux-fsdevel/20220722071228.146690-1-ebiggers@kernel.org/T/#u).

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---

v2: rebased onto man-pages master branch, mentioned xfs, and updated
    link to patchset

 man2/open.2  | 43 ++++++++++++++++++++++++++++++++-----------
 man2/statx.2 | 29 +++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+), 11 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 0326e9af0..ea38ec829 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 5.20; support varies by filesystem)
 .TE
 .in
 .PP
@@ -407,6 +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,
+f2fs, and xfs 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).

base-commit: f9f25914e4ed393ac284ab921876e8a78722c504
-- 
2.37.0

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