fs: add atomic write unit max opt to statx

XFS will be able to support large atomic writes (atomic write > 1x block)
in future. This will be achieved by using different operating methods,
depending on the size of the write.

Specifically a new method of operation based in FS atomic extent remapping
will be supported in addition to the current HW offload-based method.

The FS method will generally be appreciably slower performing than the
HW-offload method. However the FS method will be typically able to
contribute to achieving a larger atomic write unit max limit.

XFS will support a hybrid mode, where HW offload method will be used when
possible, i.e. HW offload is used when the length of the write is
supported, and for other times FS-based atomic writes will be used.

As such, there is an atomic write length at which the user may experience
appreciably slower performance.

Advertise this limit in a new statx field, stx_atomic_write_unit_max_opt.

When zero, it means that there is no such performance boundary.

Masks STATX{_ATTR}_WRITE_ATOMIC can be used to get this new field. This is
ok for older kernels which don't support this new field, as they would
report 0 in this field (from zeroing in cp_statx()) already. Furthermore
those older kernels don't support large atomic writes - apart from block
fops, but there would be consistent performance there for atomic writes
in range [unit min, unit max].

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>

block/bdev.c

@@ -1336,7 +1336,8 @@ void bdev_statx(struct path *path, struct kstat *stat,
 
 		generic_fill_statx_atomic_writes(stat,
 			queue_atomic_write_unit_min_bytes(bd_queue),
-			queue_atomic_write_unit_max_bytes(bd_queue));
+			queue_atomic_write_unit_max_bytes(bd_queue),
+			0);
 	}
 
 	stat->blksize = bdev_io_min(bdev);

fs/ext4/inode.c

@@ -5692,7 +5692,7 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
 			awu_max = sbi->s_awu_max;
 		}
 
-		generic_fill_statx_atomic_writes(stat, awu_min, awu_max);
+		generic_fill_statx_atomic_writes(stat, awu_min, awu_max, 0);
 	}
 
 	flags = ei->i_flags & EXT4_FL_USER_VISIBLE;

fs/stat.c

@@ -136,13 +136,15 @@ EXPORT_SYMBOL(generic_fill_statx_attr);
  * @stat:	Where to fill in the attribute flags
  * @unit_min:	Minimum supported atomic write length in bytes
  * @unit_max:	Maximum supported atomic write length in bytes
+ * @unit_max_opt: Optimised maximum supported atomic write length in bytes
  *
  * Fill in the STATX{_ATTR}_WRITE_ATOMIC flags in the kstat structure from
  * atomic write unit_min and unit_max values.
  */
 void generic_fill_statx_atomic_writes(struct kstat *stat,
 				      unsigned int unit_min,
-				      unsigned int unit_max)
+				      unsigned int unit_max,
+				      unsigned int unit_max_opt)
 {
 	/* Confirm that the request type is known */
 	stat->result_mask |= STATX_WRITE_ATOMIC;
@@ -153,6 +155,7 @@ void generic_fill_statx_atomic_writes(struct kstat *stat,
 	if (unit_min) {
 		stat->atomic_write_unit_min = unit_min;
 		stat->atomic_write_unit_max = unit_max;
+		stat->atomic_write_unit_max_opt = unit_max_opt;
 		/* Initially only allow 1x segment */
 		stat->atomic_write_segments_max = 1;
 
@@ -732,6 +735,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_atomic_write_unit_min = stat->atomic_write_unit_min;
 	tmp.stx_atomic_write_unit_max = stat->atomic_write_unit_max;
 	tmp.stx_atomic_write_segments_max = stat->atomic_write_segments_max;
+	tmp.stx_atomic_write_unit_max_opt = stat->atomic_write_unit_max_opt;
 
 	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
 }

fs/xfs/xfs_iops.c

@@ -610,7 +610,7 @@ xfs_report_atomic_write(
 
 	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);
+	generic_fill_statx_atomic_writes(stat, unit_min, unit_max, 0);
 }
 
 STATIC int

include/linux/fs.h

@@ -3475,7 +3475,8 @@ void generic_fillattr(struct mnt_idmap *, u32, struct inode *, struct kstat *);
 void generic_fill_statx_attr(struct inode *inode, struct kstat *stat);
 void generic_fill_statx_atomic_writes(struct kstat *stat,
 				      unsigned int unit_min,
-				      unsigned int unit_max);
+				      unsigned int unit_max,
+				      unsigned int unit_max_opt);
 extern int vfs_getattr_nosec(const struct path *, struct kstat *, u32, unsigned int);
 extern int vfs_getattr(const struct path *, struct kstat *, u32, unsigned int);
 void __inode_add_bytes(struct inode *inode, loff_t bytes);

include/linux/stat.h

@@ -57,6 +57,7 @@ struct kstat {
 	u32		dio_read_offset_align;
 	u32		atomic_write_unit_min;
 	u32		atomic_write_unit_max;
+	u32		atomic_write_unit_max_opt;
 	u32		atomic_write_segments_max;
 };
 

include/uapi/linux/stat.h

@@ -182,8 +182,12 @@ struct statx {
 	/* File offset alignment for direct I/O reads */
 	__u32	stx_dio_read_offset_align;
 
-	/* 0xb8 */
-	__u64	__spare3[9];	/* Spare space for future expansion */
+	/* Optimised max atomic write unit in bytes */
+	__u32	stx_atomic_write_unit_max_opt;
+	__u32	__spare2[1];
+
+	/* 0xc0 */
+	__u64	__spare3[8];	/* Spare space for future expansion */
 
 	/* 0x100 */
 };