block: Ensure start sector is aligned for stacking atomic writes

For stacking atomic writes, ensure that the start sector is aligned with
the device atomic write unit min and any boundary. Otherwise, we may
permit misaligned atomic writes.

Rework bdev_can_atomic_write() into a common helper to resuse the
alignment check. There also use atomic_write_hw_unit_min, which is more
proper (than atomic_write_unit_min).

Fixes: d7f36dc ("block: Support atomic writes limits for stacked devices")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20250109114000.2299896-2-john.g.garry@oracle.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: johnpgarry

block/blk-settings.c

@@ -609,14 +609,17 @@ static bool blk_stack_atomic_writes_head(struct queue_limits *t,
 }
 
 static void blk_stack_atomic_writes_limits(struct queue_limits *t,
-				struct queue_limits *b)
+				struct queue_limits *b, sector_t start)
 {
 	if (!(t->features & BLK_FEAT_ATOMIC_WRITES_STACKED))
 		goto unsupported;
 
 	if (!b->atomic_write_unit_min)
 		goto unsupported;
 
+	if (!blk_atomic_write_start_sect_aligned(start, b))
+		goto unsupported;
+
 	/*
 	 * If atomic_write_hw_max is set, we have already stacked 1x bottom
 	 * device, so check for compliance.
@@ -799,7 +802,7 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 		t->zone_write_granularity = 0;
 		t->max_zone_append_sectors = 0;
 	}
-	blk_stack_atomic_writes_limits(t, b);
+	blk_stack_atomic_writes_limits(t, b, start);
 
 	return ret;
 }

include/linux/blkdev.h

@@ -1706,6 +1706,15 @@ struct io_comp_batch {
 	void (*complete)(struct io_comp_batch *);
 };
 
+static inline bool blk_atomic_write_start_sect_aligned(sector_t sector,
+						struct queue_limits *limits)
+{
+	unsigned int alignment = max(limits->atomic_write_hw_unit_min,
+				limits->atomic_write_hw_boundary);
+
+	return IS_ALIGNED(sector, alignment >> SECTOR_SHIFT);
+}
+
 static inline bool bdev_can_atomic_write(struct block_device *bdev)
 {
 	struct request_queue *bd_queue = bdev->bd_queue;
@@ -1714,15 +1723,9 @@ static inline bool bdev_can_atomic_write(struct block_device *bdev)
 	if (!limits->atomic_write_unit_min)
 		return false;
 
-	if (bdev_is_partition(bdev)) {
-		sector_t bd_start_sect = bdev->bd_start_sect;
-		unsigned int alignment =
-			max(limits->atomic_write_unit_min,
-			    limits->atomic_write_hw_boundary);
-
-		if (!IS_ALIGNED(bd_start_sect, alignment >> SECTOR_SHIFT))
-			return false;
-	}
+	if (bdev_is_partition(bdev))
+		return blk_atomic_write_start_sect_aligned(bdev->bd_start_sect,
+							limits);
 
 	return true;
 }