add STATX_DIO_READ_ALIGN v2

add STATX_DIO_READ_ALIGN v2

[Christoph Hellwig]

Hi all,

file systems that write out of place usually require different alignment
for direct I/O writes than what they can do for reads.  This series tries
to address this by adding yet another statx field.

Changes from v1:
 - improve the comment in xfs_report_dioalign based on a contribution
   from Darrick
 - improve man page wording and formatting
 - also report the larger alignment in XFS_IOC_DIOINFO

Changes from RFC:
 - add a cleanup patch for xfs_vn_getattr
 - use xfs_inode_alloc_unitsize in xfs_vn_getattr
 - improve a comment in XFS

Diffstat:
 fs/stat.c                 |    1 
 fs/xfs/xfs_ioctl.c        |   11 ++++-
 fs/xfs/xfs_iops.c         |   62 +++++++++++++++++-----------
 include/linux/stat.h      |    1 
 include/uapi/linux/stat.h |   99 ++++++++++++++++++++++++++++++++++------------
 5 files changed, 125 insertions(+), 49 deletions(-)

[PATCH 1/5] fs: reformat the statx definition

[Christoph Hellwig]

The comments after the declaration are becoming rather unreadable with
long enough comments.  Move them into lines of their own.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
---
 include/uapi/linux/stat.h | 95 +++++++++++++++++++++++++++++----------
 1 file changed, 72 insertions(+), 23 deletions(-)

diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index 887a25286441..8b35d7d511a2 100644
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -98,43 +98,92 @@ struct statx_timestamp {
  */
 struct statx {
 	/* 0x00 */
-	__u32	stx_mask;	/* What results were written [uncond] */
-	__u32	stx_blksize;	/* Preferred general I/O size [uncond] */
-	__u64	stx_attributes;	/* Flags conveying information about the file [uncond] */
+	/* What results were written [uncond] */
+	__u32	stx_mask;
+
+	/* Preferred general I/O size [uncond] */
+	__u32	stx_blksize;
+
+	/* Flags conveying information about the file [uncond] */
+	__u64	stx_attributes;
+
 	/* 0x10 */
-	__u32	stx_nlink;	/* Number of hard links */
-	__u32	stx_uid;	/* User ID of owner */
-	__u32	stx_gid;	/* Group ID of owner */
-	__u16	stx_mode;	/* File mode */
+	/* Number of hard links */
+	__u32	stx_nlink;
+
+	/* User ID of owner */
+	__u32	stx_uid;
+
+	/* Group ID of owner */
+	__u32	stx_gid;
+
+	/* File mode */
+	__u16	stx_mode;
 	__u16	__spare0[1];
+
 	/* 0x20 */
-	__u64	stx_ino;	/* Inode number */
-	__u64	stx_size;	/* File size */
-	__u64	stx_blocks;	/* Number of 512-byte blocks allocated */
-	__u64	stx_attributes_mask; /* Mask to show what's supported in stx_attributes */
+	/* Inode number */
+	__u64	stx_ino;
+
+	/* File size */
+	__u64	stx_size;
+
+	/* Number of 512-byte blocks allocated */
+	__u64	stx_blocks;
+
+	/* Mask to show what's supported in stx_attributes */
+	__u64	stx_attributes_mask;
+
 	/* 0x40 */
-	struct statx_timestamp	stx_atime;	/* Last access time */
-	struct statx_timestamp	stx_btime;	/* File creation time */
-	struct statx_timestamp	stx_ctime;	/* Last attribute change time */
-	struct statx_timestamp	stx_mtime;	/* Last data modification time */
+	/* Last access time */
+	struct statx_timestamp	stx_atime;
+
+	/* File creation time */
+	struct statx_timestamp	stx_btime;
+
+	/* Last attribute change time */
+	struct statx_timestamp	stx_ctime;
+
+	/* Last data modification time */
+	struct statx_timestamp	stx_mtime;
+
 	/* 0x80 */
-	__u32	stx_rdev_major;	/* Device ID of special file [if bdev/cdev] */
+	/* Device ID of special file [if bdev/cdev] */
+	__u32	stx_rdev_major;
 	__u32	stx_rdev_minor;
-	__u32	stx_dev_major;	/* ID of device containing file [uncond] */
+
+	/* ID of device containing file [uncond] */
+	__u32	stx_dev_major;
 	__u32	stx_dev_minor;
+
 	/* 0x90 */
 	__u64	stx_mnt_id;
-	__u32	stx_dio_mem_align;	/* Memory buffer alignment for direct I/O */
-	__u32	stx_dio_offset_align;	/* File offset alignment for direct I/O */
+
+	/* Memory buffer alignment for direct I/O */
+	__u32	stx_dio_mem_align;
+
+	/* File offset alignment for direct I/O */
+	__u32	stx_dio_offset_align;
+
 	/* 0xa0 */
-	__u64	stx_subvol;	/* Subvolume identifier */
-	__u32	stx_atomic_write_unit_min;	/* Min atomic write unit in bytes */
-	__u32	stx_atomic_write_unit_max;	/* Max atomic write unit in bytes */
+	/* Subvolume identifier */
+	__u64	stx_subvol;
+
+	/* Min atomic write unit in bytes */
+	__u32	stx_atomic_write_unit_min;
+
+	/* Max atomic write unit in bytes */
+	__u32	stx_atomic_write_unit_max;
+
 	/* 0xb0 */
-	__u32   stx_atomic_write_segments_max;	/* Max atomic write segment count */
+	/* Max atomic write segment count */
+	__u32   stx_atomic_write_segments_max;
+
 	__u32   __spare1[1];
+
 	/* 0xb8 */
 	__u64	__spare3[9];	/* Spare space for future expansion */
+
 	/* 0x100 */
 };
 
-- 
2.45.2

[PATCH 2/5] fs: add STATX_DIO_READ_ALIGN

[Christoph Hellwig]

Add a separate dio read align field, as many out of place write
file systems can easily do reads aligned to the device sector size,
but require bigger alignment for writes.

This is usually papered over by falling back to buffered I/O for smaller
writes and doing read-modify-write cycles, but performance for this
sucks, so applications benefit from knowing the actual write alignment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/stat.c                 | 1 +
 include/linux/stat.h      | 1 +
 include/uapi/linux/stat.h | 4 +++-
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/stat.c b/fs/stat.c
index 0870e969a8a0..2c0e111a098a 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -725,6 +725,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_mnt_id = stat->mnt_id;
 	tmp.stx_dio_mem_align = stat->dio_mem_align;
 	tmp.stx_dio_offset_align = stat->dio_offset_align;
+	tmp.stx_dio_read_offset_align = stat->dio_read_offset_align;
 	tmp.stx_subvol = stat->subvol;
 	tmp.stx_atomic_write_unit_min = stat->atomic_write_unit_min;
 	tmp.stx_atomic_write_unit_max = stat->atomic_write_unit_max;
diff --git a/include/linux/stat.h b/include/linux/stat.h
index 3d900c86981c..9d8382e23a9c 100644
--- a/include/linux/stat.h
+++ b/include/linux/stat.h
@@ -52,6 +52,7 @@ struct kstat {
 	u64		mnt_id;
 	u32		dio_mem_align;
 	u32		dio_offset_align;
+	u32		dio_read_offset_align;
 	u64		change_cookie;
 	u64		subvol;
 	u32		atomic_write_unit_min;
diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index 8b35d7d511a2..f78ee3670dd5 100644
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -179,7 +179,8 @@ struct statx {
 	/* Max atomic write segment count */
 	__u32   stx_atomic_write_segments_max;
 
-	__u32   __spare1[1];
+	/* File offset alignment for direct I/O reads */
+	__u32	stx_dio_read_offset_align;
 
 	/* 0xb8 */
 	__u64	__spare3[9];	/* Spare space for future expansion */
@@ -213,6 +214,7 @@ struct statx {
 #define STATX_MNT_ID_UNIQUE	0x00004000U	/* Want/got extended stx_mount_id */
 #define STATX_SUBVOL		0x00008000U	/* Want/got stx_subvol */
 #define STATX_WRITE_ATOMIC	0x00010000U	/* Want/got atomic_write_* fields */
+#define STATX_DIO_READ_ALIGN	0x00020000U	/* Want/got dio read alignment info */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
-- 
2.45.2

[PATCH 3/5] xfs: cleanup xfs_vn_getattr

[Christoph Hellwig]

Split the two bits of optional statx reporting into their own helpers
so that they are self-contained instead of deeply indented in the main
getattr handler.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/xfs/xfs_iops.c | 47 +++++++++++++++++++++++------------------------
 1 file changed, 23 insertions(+), 24 deletions(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 207e0dadffc3..6b0228a21617 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -573,17 +573,28 @@ xfs_stat_blksize(
 }
 
 static void
-xfs_get_atomic_write_attr(
+xfs_report_dioalign(
 	struct xfs_inode	*ip,
-	unsigned int		*unit_min,
-	unsigned int		*unit_max)
+	struct kstat		*stat)
 {
-	if (!xfs_inode_can_atomicwrite(ip)) {
-		*unit_min = *unit_max = 0;
-		return;
-	}
+	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
+	struct block_device	*bdev = target->bt_bdev;
 
-	*unit_min = *unit_max = ip->i_mount->m_sb.sb_blocksize;
+	stat->result_mask |= STATX_DIOALIGN;
+	stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
+	stat->dio_offset_align = bdev_logical_block_size(bdev);
+}
+
+static void
+xfs_report_atomic_write(
+	struct xfs_inode	*ip,
+	struct kstat		*stat)
+{
+	unsigned int		unit_min = 0, unit_max = 0;
+
+	if (xfs_inode_can_atomicwrite(ip))
+		unit_min = unit_max = ip->i_mount->m_sb.sb_blocksize;
+	generic_fill_statx_atomic_writes(stat, unit_min, unit_max);
 }
 
 STATIC int
@@ -647,22 +658,10 @@ xfs_vn_getattr(
 		stat->rdev = inode->i_rdev;
 		break;
 	case S_IFREG:
-		if (request_mask & STATX_DIOALIGN) {
-			struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
-			struct block_device	*bdev = target->bt_bdev;
-
-			stat->result_mask |= STATX_DIOALIGN;
-			stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
-			stat->dio_offset_align = bdev_logical_block_size(bdev);
-		}
-		if (request_mask & STATX_WRITE_ATOMIC) {
-			unsigned int	unit_min, unit_max;
-
-			xfs_get_atomic_write_attr(ip, &unit_min,
-					&unit_max);
-			generic_fill_statx_atomic_writes(stat,
-					unit_min, unit_max);
-		}
+		if (request_mask & STATX_DIOALIGN)
+			xfs_report_dioalign(ip, stat);
+		if (request_mask & STATX_WRITE_ATOMIC)
+			xfs_report_atomic_write(ip, stat);
 		fallthrough;
 	default:
 		stat->blksize = xfs_stat_blksize(ip);
-- 
2.45.2

[PATCH 4/5] xfs: report the correct read/write dio alignment for reflinked inodes

[Christoph Hellwig]

For I/O to reflinked blocks we always need to write an entire new
file system block, and the code enforces the file system block alignment
for the entire file if it has any reflinked blocks.

Use the new STATX_DIO_READ_ALIGN flag to report the asymmetric read
vs write alignments for reflinked files.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/xfs/xfs_iops.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 6b0228a21617..40289fe6f5b2 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -580,9 +580,24 @@ xfs_report_dioalign(
 	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
 	struct block_device	*bdev = target->bt_bdev;
 
-	stat->result_mask |= STATX_DIOALIGN;
+	stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
 	stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
-	stat->dio_offset_align = bdev_logical_block_size(bdev);
+
+	/*
+	 * For COW inodes, we can only perform out of place writes of entire
+	 * allocation units (blocks or RT extents).
+	 * For writes smaller than the allocation unit, we must fall back to
+	 * buffered I/O to perform read-modify-write cycles.  At best this is
+	 * highly inefficient; at worst it leads to page cache invalidation
+	 * races.  Tell applications to avoid this by reporting the larger write
+	 * alignment in dio_offset_align, and the smaller read alignment in
+	 * dio_read_offset_align.
+	 */
+	stat->dio_read_offset_align = bdev_logical_block_size(bdev);
+	if (xfs_is_cow_inode(ip))
+		stat->dio_offset_align = xfs_inode_alloc_unitsize(ip);
+	else
+		stat->dio_offset_align = stat->dio_read_offset_align;
 }
 
 static void
@@ -658,7 +673,7 @@ xfs_vn_getattr(
 		stat->rdev = inode->i_rdev;
 		break;
 	case S_IFREG:
-		if (request_mask & STATX_DIOALIGN)
+		if (request_mask & (STATX_DIOALIGN | STATX_DIO_READ_ALIGN))
 			xfs_report_dioalign(ip, stat);
 		if (request_mask & STATX_WRITE_ATOMIC)
 			xfs_report_atomic_write(ip, stat);
-- 
2.45.2

[PATCH 5/5] xfs: report larger dio alignment for COW inodes

[Christoph Hellwig]

For I/O to reflinked blocks we always need to write an entire new file
system block, and the code enforces the file system block alignment for
the entire file if it has any reflinked blocks.  Mirror the larger
value reported in the statx in the dio_offset_align in the xfs-specific
XFS_IOC_DIOINFO ioctl for the same reason.

Don't bother adding a new field for the read alignment to this legacy
ioctl as all new users should use statx instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_ioctl.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 0789c18aaa18..20f3cf5391c6 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1204,7 +1204,16 @@ xfs_file_ioctl(
 		struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
 		struct dioattr		da;
 
-		da.d_mem =  da.d_miniosz = target->bt_logical_sectorsize;
+		da.d_mem = target->bt_logical_sectorsize;
+
+		/*
+		 * See xfs_report_dioalign() why report a potential larger than
+		 * sector sizevalue here for COW inodes.
+		 */
+		if (xfs_is_cow_inode(ip))
+			da.d_miniosz = xfs_inode_alloc_unitsize(ip);
+		else
+			da.d_miniosz = target->bt_logical_sectorsize;
 		da.d_maxiosz = INT_MAX & ~(da.d_miniosz - 1);
 
 		if (copy_to_user(arg, &da, sizeof(da)))
-- 
2.45.2

[PATCH] statx.2: document STATX_DIO_READ_ALIGN

[Christoph Hellwig]

Document the new STATX_DIO_READ_ALIGN flag and the new
stx_dio_read_offset_align field guarded by it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 man/man2/statx.2 | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/man/man2/statx.2 b/man/man2/statx.2
index c5b5a28ec2f1..8ef6a1cfb1c0 100644
--- a/man/man2/statx.2
+++ b/man/man2/statx.2
@@ -76,6 +76,9 @@ struct statx {
     __u32 stx_atomic_write_unit_min;
     __u32 stx_atomic_write_unit_max;
     __u32 stx_atomic_write_segments_max;
+
+    /* File offset alignment for direct I/O reads */
+    __u32   stx_dio_read_offset_align;
 };
 .EE
 .in
@@ -261,7 +264,7 @@ 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
+STATX_DIOALIGN	Want stx_dio_mem_align and stx_dio_offset_align.
 	(since Linux 6.1; support varies by filesystem)
 STATX_MNT_ID_UNIQUE	Want unique stx_mnt_id (since Linux 6.8)
 STATX_SUBVOL	Want stx_subvol
@@ -270,6 +273,8 @@ STATX_WRITE_ATOMIC	Want stx_atomic_write_unit_min,
 	stx_atomic_write_unit_max,
 	and stx_atomic_write_segments_max.
 	(since Linux 6.11; support varies by filesystem)
+STATX_DIO_READ_ALIGN	Want stx_dio_read_offset_align.
+	(since Linux 6.14; support varies by filesystem)
 .TE
 .in
 .P
@@ -467,6 +472,25 @@ This will only be nonzero if
 .I stx_dio_mem_align
 is nonzero, and vice versa.
 .TP
+.I stx_dio_read_offset_align
+The alignment (in bytes) required for file offsets and I/O segment lengths for
+direct I/O reads
+.RB ( O_DIRECT )
+on this file.
+If zero the limit in
+.I stx_dio_offset_align
+applies for reads as well.
+If non-zero this value must be smaller than
+.I stx_dio_offset_align
+which must be provided by the file system.
+The memory alignment in
+.I stx_dio_mem_align
+is not affected by this value.
+.IP
+.B STATX_DIO_READ_ALIGN
+.RI ( stx_dio_offset_align )
+is supported by xfs on regular files since Linux 6.14.
+.TP
 .I stx_subvol
 Subvolume number of the current file.
 .IP
-- 
2.45.2

Re: [PATCH 1/5] fs: reformat the statx definition

[John Garry]

On 08/01/2025 08:55, Christoph Hellwig wrote:
> The comments after the declaration are becoming rather unreadable with
> long enough comments.  Move them into lines of their own.
> 
> Signed-off-by: Christoph Hellwig<hch@lst.de>
> Reviewed-by: Jan Kara<jack@suse.cz>
> Reviewed-by: Darrick J. Wong<djwong@kernel.org>
> ---

FWIW:
Reviewed-by: John Garry <john.g.garry@oracle.com>

Re: [PATCH 2/5] fs: add STATX_DIO_READ_ALIGN

[John Garry]

On 08/01/2025 08:55, Christoph Hellwig wrote:
> Add a separate dio read align field, as many out of place write
> file systems can easily do reads aligned to the device sector size,
> but require bigger alignment for writes.
> 
> This is usually papered over by falling back to buffered I/O for smaller
> writes and doing read-modify-write cycles, but performance for this
> sucks, so applications benefit from knowing the actual write alignment.
> 
> Signed-off-by: Christoph Hellwig<hch@lst.de>
> Reviewed-by: Jan Kara<jack@suse.cz>
> Reviewed-by: Darrick J. Wong<djwong@kernel.org>


FWIW:
Reviewed-by: John Garry <john.g.garry@oracle.com>

Re: [PATCH 4/5] xfs: report the correct read/write dio alignment for reflinked inodes

[John Garry]

On 08/01/2025 08:55, Christoph Hellwig wrote:
> For I/O to reflinked blocks we always need to write an entire new
> file system block, and the code enforces the file system block alignment
> for the entire file if it has any reflinked blocks.
> 
> Use the new STATX_DIO_READ_ALIGN flag to report the asymmetric read
> vs write alignments for reflinked files.
> 
> Signed-off-by: Christoph Hellwig<hch@lst.de>
> Reviewed-by: "Darrick J. Wong"<djwong@kernel.org>


FWIW:
Reviewed-by: John Garry <john.g.garry@oracle.com>

Re: [PATCH 5/5] xfs: report larger dio alignment for COW inodes

[John Garry]

On 08/01/2025 08:55, Christoph Hellwig wrote:
> For I/O to reflinked blocks we always need to write an entire new file
> system block, and the code enforces the file system block alignment for
> the entire file if it has any reflinked blocks.  Mirror the larger
> value reported in the statx in the dio_offset_align in the xfs-specific
> XFS_IOC_DIOINFO ioctl for the same reason.
> 
> Don't bother adding a new field for the read alignment to this legacy
> ioctl as all new users should use statx instead.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   fs/xfs/xfs_ioctl.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 0789c18aaa18..20f3cf5391c6 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1204,7 +1204,16 @@ xfs_file_ioctl(
>   		struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
>   		struct dioattr		da;
>   
> -		da.d_mem =  da.d_miniosz = target->bt_logical_sectorsize;
> +		da.d_mem = target->bt_logical_sectorsize;
> +
> +		/*
> +		 * See xfs_report_dioalign() why report a potential larger than
> +		 * sector sizevalue here for COW inodes.

nit: sizevalue

> +		 */
> +		if (xfs_is_cow_inode(ip))
> +			da.d_miniosz = xfs_inode_alloc_unitsize(ip);
> +		else
> +			da.d_miniosz = target->bt_logical_sectorsize;
>   		da.d_maxiosz = INT_MAX & ~(da.d_miniosz - 1);
>   
>   		if (copy_to_user(arg, &da, sizeof(da)))

Re: [PATCH 4/5] xfs: report the correct read/write dio alignment for reflinked inodes

[John Garry]

On 08/01/2025 08:55, Christoph Hellwig wrote:
> @@ -580,9 +580,24 @@ xfs_report_dioalign(
>   	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
>   	struct block_device	*bdev = target->bt_bdev;
>   
> -	stat->result_mask |= STATX_DIOALIGN;
> +	stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;

BTW, it would be a crappy userspace which can't handle fields which it 
did not ask for, e.g. asked for STATX_DIOALIGN, but got  STATX_DIOALIGN 
and STATX_DIO_READ_ALIGN

Re: [PATCH 4/5] xfs: report the correct read/write dio alignment for reflinked inodes

[Christoph Hellwig]

On Wed, Jan 08, 2025 at 10:13:02AM +0000, John Garry wrote:
> On 08/01/2025 08:55, Christoph Hellwig wrote:
>> @@ -580,9 +580,24 @@ xfs_report_dioalign(
>>   	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
>>   	struct block_device	*bdev = target->bt_bdev;
>>   -	stat->result_mask |= STATX_DIOALIGN;
>> +	stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
>
> BTW, it would be a crappy userspace which can't handle fields which it did 
> not ask for, e.g. asked for STATX_DIOALIGN, but got  STATX_DIOALIGN and 
> STATX_DIO_READ_ALIGN

Well, the space is marked for extension.  I don't think there ever
was a requirement only fields asked for are reported, but if that
feels safer I could switch to that.

Re: [PATCH 4/5] xfs: report the correct read/write dio alignment for reflinked inodes

[Darrick J. Wong]

On Wed, Jan 08, 2025 at 04:18:27PM +0100, Christoph Hellwig wrote:
> On Wed, Jan 08, 2025 at 10:13:02AM +0000, John Garry wrote:
> > On 08/01/2025 08:55, Christoph Hellwig wrote:
> >> @@ -580,9 +580,24 @@ xfs_report_dioalign(
> >>   	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> >>   	struct block_device	*bdev = target->bt_bdev;
> >>   -	stat->result_mask |= STATX_DIOALIGN;
> >> +	stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
> >
> > BTW, it would be a crappy userspace which can't handle fields which it did 
> > not ask for, e.g. asked for STATX_DIOALIGN, but got  STATX_DIOALIGN and 
> > STATX_DIO_READ_ALIGN
> 
> Well, the space is marked for extension.  I don't think there ever
> was a requirement only fields asked for are reported, but if that
> feels safer I could switch to that.

From the "Returned information" section of the statx manpage:

"It should be noted that the kernel may return fields that weren't
requested and may fail to return fields that were requested, depending
on what the backing filesystem supports.  (Fields that are given values
despite being unrequested can just be ignored.)  In either case,
stx_mask will not be equal mask.

<snip>

"A filesystem may also fill in fields that the caller didn't ask for if
it has values for them available and the information is available at no
extra cost.  If this happens,  the corresponding bits will be set in
stx_mask."

In other words, the kernel is allowed to return information/flags that
weren't requested, and userspace can ignore it.

--D

Re: [PATCH 5/5] xfs: report larger dio alignment for COW inodes

[Darrick J. Wong]

On Wed, Jan 08, 2025 at 10:11:27AM +0000, John Garry wrote:
> On 08/01/2025 08:55, Christoph Hellwig wrote:
> > For I/O to reflinked blocks we always need to write an entire new file
> > system block, and the code enforces the file system block alignment for
> > the entire file if it has any reflinked blocks.  Mirror the larger
> > value reported in the statx in the dio_offset_align in the xfs-specific
> > XFS_IOC_DIOINFO ioctl for the same reason.
> > 
> > Don't bother adding a new field for the read alignment to this legacy
> > ioctl as all new users should use statx instead.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> >   fs/xfs/xfs_ioctl.c | 11 ++++++++++-
> >   1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > index 0789c18aaa18..20f3cf5391c6 100644
> > --- a/fs/xfs/xfs_ioctl.c
> > +++ b/fs/xfs/xfs_ioctl.c
> > @@ -1204,7 +1204,16 @@ xfs_file_ioctl(
> >   		struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> >   		struct dioattr		da;
> > -		da.d_mem =  da.d_miniosz = target->bt_logical_sectorsize;
> > +		da.d_mem = target->bt_logical_sectorsize;
> > +
> > +		/*
> > +		 * See xfs_report_dioalign() why report a potential larger than
> > +		 * sector sizevalue here for COW inodes.
> 
> nit: sizevalue

The sentence reads oddly to me; how about:

"See xfs_report_dioalign() for an explanation about why we report a
value larger than the sector size for COW inodes."?

The code change looks ok though.
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> > +		 */
> > +		if (xfs_is_cow_inode(ip))
> > +			da.d_miniosz = xfs_inode_alloc_unitsize(ip);
> > +		else
> > +			da.d_miniosz = target->bt_logical_sectorsize;
> >   		da.d_maxiosz = INT_MAX & ~(da.d_miniosz - 1);
> >   		if (copy_to_user(arg, &da, sizeof(da)))
> 
> 

Re: [PATCH] statx.2: document STATX_DIO_READ_ALIGN

[Darrick J. Wong]

On Wed, Jan 08, 2025 at 09:59:00AM +0100, Christoph Hellwig wrote:
> Document the new STATX_DIO_READ_ALIGN flag and the new
> stx_dio_read_offset_align field guarded by it.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  man/man2/statx.2 | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/man/man2/statx.2 b/man/man2/statx.2
> index c5b5a28ec2f1..8ef6a1cfb1c0 100644
> --- a/man/man2/statx.2
> +++ b/man/man2/statx.2
> @@ -76,6 +76,9 @@ struct statx {
>      __u32 stx_atomic_write_unit_min;
>      __u32 stx_atomic_write_unit_max;
>      __u32 stx_atomic_write_segments_max;
> +
> +    /* File offset alignment for direct I/O reads */
> +    __u32   stx_dio_read_offset_align;
>  };
>  .EE
>  .in
> @@ -261,7 +264,7 @@ 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
> +STATX_DIOALIGN	Want stx_dio_mem_align and stx_dio_offset_align.
>  	(since Linux 6.1; support varies by filesystem)
>  STATX_MNT_ID_UNIQUE	Want unique stx_mnt_id (since Linux 6.8)
>  STATX_SUBVOL	Want stx_subvol
> @@ -270,6 +273,8 @@ STATX_WRITE_ATOMIC	Want stx_atomic_write_unit_min,
>  	stx_atomic_write_unit_max,
>  	and stx_atomic_write_segments_max.
>  	(since Linux 6.11; support varies by filesystem)
> +STATX_DIO_READ_ALIGN	Want stx_dio_read_offset_align.
> +	(since Linux 6.14; support varies by filesystem)
>  .TE
>  .in
>  .P
> @@ -467,6 +472,25 @@ This will only be nonzero if
>  .I stx_dio_mem_align
>  is nonzero, and vice versa.
>  .TP
> +.I stx_dio_read_offset_align
> +The alignment (in bytes) required for file offsets and I/O segment lengths for
> +direct I/O reads
> +.RB ( O_DIRECT )
> +on this file.
> +If zero the limit in

nit: add a comma here (really a dependent clause) to make it clearer
that 'zero' isn't being used as a verb here:

"If zero, the limit in..."

> +.I stx_dio_offset_align
> +applies for reads as well.
> +If non-zero this value must be smaller than

Same here.

"If non-zero, this value..."

With that fixed,
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> +.I stx_dio_offset_align
> +which must be provided by the file system.
> +The memory alignment in
> +.I stx_dio_mem_align
> +is not affected by this value.
> +.IP
> +.B STATX_DIO_READ_ALIGN
> +.RI ( stx_dio_offset_align )
> +is supported by xfs on regular files since Linux 6.14.
> +.TP
>  .I stx_subvol
>  Subvolume number of the current file.
>  .IP
> -- 
> 2.45.2
> 
> 

Re: [PATCH 4/5] xfs: report the correct read/write dio alignment for reflinked inodes

[Eric Biggers]

On Wed, Jan 08, 2025 at 09:55:32AM +0100, Christoph Hellwig wrote:
> For I/O to reflinked blocks we always need to write an entire new
> file system block, and the code enforces the file system block alignment
> for the entire file if it has any reflinked blocks.
> 
> Use the new STATX_DIO_READ_ALIGN flag to report the asymmetric read
> vs write alignments for reflinked files.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> ---
>  fs/xfs/xfs_iops.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 6b0228a21617..40289fe6f5b2 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -580,9 +580,24 @@ xfs_report_dioalign(
>  	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
>  	struct block_device	*bdev = target->bt_bdev;
>  
> -	stat->result_mask |= STATX_DIOALIGN;
> +	stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
>  	stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
> -	stat->dio_offset_align = bdev_logical_block_size(bdev);
> +
> +	/*
> +	 * For COW inodes, we can only perform out of place writes of entire
> +	 * allocation units (blocks or RT extents).
> +	 * For writes smaller than the allocation unit, we must fall back to
> +	 * buffered I/O to perform read-modify-write cycles.  At best this is
> +	 * highly inefficient; at worst it leads to page cache invalidation
> +	 * races.  Tell applications to avoid this by reporting the larger write
> +	 * alignment in dio_offset_align, and the smaller read alignment in
> +	 * dio_read_offset_align.
> +	 */
> +	stat->dio_read_offset_align = bdev_logical_block_size(bdev);
> +	if (xfs_is_cow_inode(ip))
> +		stat->dio_offset_align = xfs_inode_alloc_unitsize(ip);
> +	else
> +		stat->dio_offset_align = stat->dio_read_offset_align;

This contradicts the proposed man page, which says the following about
stx_dio_read_offset_align offset:

          If non-zero this value must be smaller than  stx_dio_offset_align
          which  must be provided by the file system.

The proposed code sets stx_dio_read_offset_align and stx_dio_offset_align to the
same value in some cases.

Perhaps the documentation should say "less than or equal to"?

Also, the claim that stx_dio_offset_align "must be provided by the file system"
if stx_dio_read_offset_align is nonzero should probably be conditional on
STATX_DIOALIGN being provided too.

- Eric

Re: [PATCH 4/5] xfs: report the correct read/write dio alignment for reflinked inodes

[Christoph Hellwig]

On Wed, Jan 08, 2025 at 09:53:58AM -0800, Eric Biggers wrote:
> This contradicts the proposed man page, which says the following about
> stx_dio_read_offset_align offset:
> 
>           If non-zero this value must be smaller than  stx_dio_offset_align
>           which  must be provided by the file system.
> 
> The proposed code sets stx_dio_read_offset_align and stx_dio_offset_align to the
> same value in some cases.
> 
> Perhaps the documentation should say "less than or equal to"?

Yes, I'll fix it up.

> Also, the claim that stx_dio_offset_align "must be provided by the file system"
> if stx_dio_read_offset_align is nonzero should probably be conditional on
> STATX_DIOALIGN being provided too.

I'll add a "if requested" to the man page.

[PATCH 2/5] fs: add STATX_DIO_READ_ALIGN

[Christoph Hellwig]

Add a separate dio read align field, as many out of place write
file systems can easily do reads aligned to the device sector size,
but require bigger alignment for writes.

This is usually papered over by falling back to buffered I/O for smaller
writes and doing read-modify-write cycles, but performance for this
sucks, so applications benefit from knowing the actual write alignment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/stat.c                 | 1 +
 include/linux/stat.h      | 1 +
 include/uapi/linux/stat.h | 4 +++-
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/stat.c b/fs/stat.c
index 0870e969a8a0..2c0e111a098a 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -725,6 +725,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_mnt_id = stat->mnt_id;
 	tmp.stx_dio_mem_align = stat->dio_mem_align;
 	tmp.stx_dio_offset_align = stat->dio_offset_align;
+	tmp.stx_dio_read_offset_align = stat->dio_read_offset_align;
 	tmp.stx_subvol = stat->subvol;
 	tmp.stx_atomic_write_unit_min = stat->atomic_write_unit_min;
 	tmp.stx_atomic_write_unit_max = stat->atomic_write_unit_max;
diff --git a/include/linux/stat.h b/include/linux/stat.h
index 3d900c86981c..9d8382e23a9c 100644
--- a/include/linux/stat.h
+++ b/include/linux/stat.h
@@ -52,6 +52,7 @@ struct kstat {
 	u64		mnt_id;
 	u32		dio_mem_align;
 	u32		dio_offset_align;
+	u32		dio_read_offset_align;
 	u64		change_cookie;
 	u64		subvol;
 	u32		atomic_write_unit_min;
diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index 8b35d7d511a2..f78ee3670dd5 100644
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -179,7 +179,8 @@ struct statx {
 	/* Max atomic write segment count */
 	__u32   stx_atomic_write_segments_max;
 
-	__u32   __spare1[1];
+	/* File offset alignment for direct I/O reads */
+	__u32	stx_dio_read_offset_align;
 
 	/* 0xb8 */
 	__u64	__spare3[9];	/* Spare space for future expansion */
@@ -213,6 +214,7 @@ struct statx {
 #define STATX_MNT_ID_UNIQUE	0x00004000U	/* Want/got extended stx_mount_id */
 #define STATX_SUBVOL		0x00008000U	/* Want/got stx_subvol */
 #define STATX_WRITE_ATOMIC	0x00010000U	/* Want/got atomic_write_* fields */
+#define STATX_DIO_READ_ALIGN	0x00020000U	/* Want/got dio read alignment info */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
-- 
2.45.2