[PATCH RFC 07/16] block: Add bdev_find_max_atomic_write_alignment()

[John Garry <john.g.garry@oracle.com>]

Add a function to find the max alignment of an atomic write for a bdev
when provided with an offset and length.

We should be able to optimise this function later, most especially since
the values involved are powers-of-2.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 block/bdev.c           | 39 +++++++++++++++++++++++++++++++++++++++
 include/linux/blkdev.h |  9 +++++++++
 2 files changed, 48 insertions(+)

diff --git a/block/bdev.c b/block/bdev.c
index 6a5fd5abaadc..3373f0d5cad9 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -46,6 +46,45 @@ struct block_device *I_BDEV(struct inode *inode)
 }
 EXPORT_SYMBOL(I_BDEV);
 
+unsigned int bdev_find_max_atomic_write_alignment(struct block_device *bdev,
+					loff_t pos, unsigned int len)
+{
+	struct request_queue *bd_queue = bdev->bd_queue;
+	struct queue_limits *limits = &bd_queue->limits;
+	unsigned int atomic_write_unit_min = limits->atomic_write_unit_min;
+	unsigned int atomic_write_unit_max = limits->atomic_write_unit_max;
+	unsigned int max_align;
+
+	pos /= SECTOR_SIZE;
+	len /= SECTOR_SIZE;
+
+	max_align = min_not_zero(len, atomic_write_unit_max);
+
+	if (len <= 1)
+		return atomic_write_unit_min * SECTOR_SIZE;
+
+	max_align = rounddown_pow_of_two(max_align);
+	while (1) {
+		unsigned int mod1, mod2;
+
+		if (max_align == 0)
+			return atomic_write_unit_min * SECTOR_SIZE;
+
+		/* This should not happen */
+		if (!is_power_of_2(max_align))
+			goto end;
+
+		mod1 = len % max_align;
+		mod2 = pos % max_align;
+		if (!mod1 && !mod2)
+			break;
+end:
+		max_align /= 2;
+	}
+
+	return max_align * SECTOR_SIZE;
+}
+
 static void bdev_write_inode(struct block_device *bdev)
 {
 	struct inode *inode = bdev->bd_inode;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 19d33b2897b2..96138550928c 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1529,6 +1529,8 @@ void sync_bdevs(bool wait);
 void bdev_statx_dioalign(struct inode *inode, struct kstat *stat);
 void bdev_statx_atomic(struct inode *inode, struct kstat *stat);
 void printk_all_partitions(void);
+unsigned int bdev_find_max_atomic_write_alignment(struct block_device *bdev,
+				loff_t pos, unsigned int len);
 #else
 static inline void invalidate_bdev(struct block_device *bdev)
 {
@@ -1553,6 +1555,13 @@ static inline void bdev_statx_atomic(struct inode *inode, struct kstat *stat)
 static inline void printk_all_partitions(void)
 {
 }
+static inline unsigned int bdev_find_max_atomic_write_alignment(
+				struct block_device *bdev,
+				loff_t pos, unsigned int len)
+{
+	return 0;
+}		
+				
 #endif /* CONFIG_BLOCK */
 
 int fsync_bdev(struct block_device *bdev);
-- 
2.31.1