xfs: fix number of GC bvecs

GC scratch allocations can wrap around and use the same buffer twice, and
the current code fails to account for that.  So far this worked due to
rounding in the block layer, but changes to the bio allocator drop the
over-provisioning and generic/256 or generic/361 will now usually fail
when running against the current block tree.

Simplify the allocation to always pass the maximum value that is easier to
verify, as a saving of up to one bvec per allocation isn't worth the
effort to verify a complicated calculated value.

Fixes: 102f444 ("xfs: rework zone GC buffer management")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Link: https://patch.msgid.link/20260407140538.633364-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

fs/xfs/xfs_zone_gc.c

@@ -671,7 +671,6 @@ xfs_zone_gc_start_chunk(
 	struct xfs_inode	*ip;
 	struct bio		*bio;
 	xfs_daddr_t		daddr;
-	unsigned int		len;
 	bool			is_seq;
 
 	if (xfs_is_shutdown(mp))
@@ -686,15 +685,16 @@ xfs_zone_gc_start_chunk(
 		return false;
 	}
 
-	len = XFS_FSB_TO_B(mp, irec.rm_blockcount);
-	bio = bio_alloc_bioset(bdev,
-			min(howmany(len, XFS_GC_BUF_SIZE) + 1, XFS_GC_NR_BUFS),
-			REQ_OP_READ, GFP_NOFS, &data->bio_set);
-
+	/*
+	 * Scratch allocation can wrap around to the same buffer again,
+	 * provision an extra bvec for that case.
+	 */
+	bio = bio_alloc_bioset(bdev, XFS_GC_NR_BUFS + 1, REQ_OP_READ, GFP_NOFS,
+			&data->bio_set);
 	chunk = container_of(bio, struct xfs_gc_bio, bio);
 	chunk->ip = ip;
 	chunk->offset = XFS_FSB_TO_B(mp, irec.rm_offset);
-	chunk->len = len;
+	chunk->len = XFS_FSB_TO_B(mp, irec.rm_blockcount);
 	chunk->old_startblock =
 		xfs_rgbno_to_rtb(iter->victim_rtg, irec.rm_startblock);
 	chunk->new_daddr = daddr;
@@ -708,8 +708,9 @@ xfs_zone_gc_start_chunk(
 	bio->bi_iter.bi_sector = xfs_rtb_to_daddr(mp, chunk->old_startblock);
 	bio->bi_end_io = xfs_zone_gc_end_io;
 	xfs_zone_gc_add_data(chunk);
-	data->scratch_head = (data->scratch_head + len) % data->scratch_size;
-	data->scratch_available -= len;
+	data->scratch_head =
+		(data->scratch_head + chunk->len) % data->scratch_size;
+	data->scratch_available -= chunk->len;
 
 	XFS_STATS_INC(mp, xs_gc_read_calls);