xfs: set s_min_writeback_pages for zoned file systems

Set s_min_writeback_pages to the zone size, so that writeback always
writes up to a full zone.  This ensures that writeback does not add
spurious file fragmentation when writing back a large number of
files that are larger than the zone size.

Fixes: 4e4d520 ("xfs: add the zoned space allocator")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251017034611.651385-4-hch@lst.de
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Christoph Hellwig

fs/xfs/xfs_zone_alloc.c

@@ -1215,6 +1215,7 @@ xfs_mount_zones(
 		.mp		= mp,
 	};
 	struct xfs_buftarg	*bt = mp->m_rtdev_targp;
+	xfs_extlen_t		zone_blocks = mp->m_groups[XG_TYPE_RTG].blocks;
 	int			error;
 
 	if (!bt) {
@@ -1245,10 +1246,33 @@ xfs_mount_zones(
 		return -ENOMEM;
 
 	xfs_info(mp, "%u zones of %u blocks (%u max open zones)",
-		 mp->m_sb.sb_rgcount, mp->m_groups[XG_TYPE_RTG].blocks,
-		 mp->m_max_open_zones);
+		 mp->m_sb.sb_rgcount, zone_blocks, mp->m_max_open_zones);
 	trace_xfs_zones_mount(mp);
 
+	/*
+	 * The writeback code switches between inodes regularly to provide
+	 * fairness.  The default lower bound is 4MiB, but for zoned file
+	 * systems we want to increase that both to reduce seeks, but also more
+	 * importantly so that workloads that writes files in a multiple of the
+	 * zone size do not get fragmented and require garbage collection when
+	 * they shouldn't.  Increase is to the zone size capped by the max
+	 * extent len.
+	 *
+	 * Note that because s_min_writeback_pages is a superblock field, this
+	 * value also get applied to non-zoned files on the data device if
+	 * there are any.  On typical zoned setup all data is on the RT device
+	 * because using the more efficient sequential write required zones
+	 * is the reason for using the zone allocator, and either the RT device
+	 * and the (meta)data device are on the same block device, or the
+	 * (meta)data device is on a fast SSD while the data on the RT device
+	 * is on a SMR HDD.  In any combination of the above cases enforcing
+	 * the higher min_writeback_pages for non-RT inodes is either a noop
+	 * or beneficial.
+	 */
+	mp->m_super->s_min_writeback_pages =
+		XFS_FSB_TO_B(mp, min(zone_blocks, XFS_MAX_BMBT_EXTLEN)) >>
+			PAGE_SHIFT;
+
 	if (bdev_is_zoned(bt->bt_bdev)) {
 		error = blkdev_report_zones(bt->bt_bdev,
 				XFS_FSB_TO_BB(mp, mp->m_sb.sb_rtstart),