[PATCH] Btrfs: fix max chunk size on raid5/6

[Chris Mason <chris.mason@fusionio.com>]

Hi everyone,

This spot in the chunk allocation code has seen a lot of little tweaks,
so I wanted to send this patch out for more eyes.

--

We try to limit the size of a chunk to 10GB, which keeps the unit of
work reasonable during balance and resize operations.  The limit checks
were taking into account the number of copies of the data we had but
what they really should be doing is comparing against the logical
size of the chunk we're creating.

This moves the code around a little to use the count of data stripes
from raid5/6.

Signed-off-by: Chris Mason <chris.mason@fusionio.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 5d6010b..538c5cf 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3837,10 +3837,6 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 	 */
 	data_stripes = num_stripes / ncopies;
 
-	if (stripe_size * ndevs > max_chunk_size * ncopies) {
-		stripe_size = max_chunk_size * ncopies;
-		do_div(stripe_size, ndevs);
-	}
 	if (type & BTRFS_BLOCK_GROUP_RAID5) {
 		raid_stripe_len = find_raid56_stripe_len(ndevs - 1,
 				 btrfs_super_stripesize(info->super_copy));
@@ -3851,6 +3847,27 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 				 btrfs_super_stripesize(info->super_copy));
 		data_stripes = num_stripes - 2;
 	}
+
+	/*
+	 * Use the number of data stripes to figure out how big this chunk
+	 * is really going to be in terms of logical address space,
+	 * and compare that answer with the max chunk size
+	 */
+	if (stripe_size * data_stripes > max_chunk_size) {
+		u64 mask = (1ULL << 24) - 1;
+		stripe_size = max_chunk_size;
+		do_div(stripe_size, data_stripes);
+
+		/* bump the answer up to a 16MB boundary */
+		stripe_size = (stripe_size + mask) & ~mask;
+
+		/* but don't go higher than the limits we found
+		 * while searching for free extents
+		 */
+		if (stripe_size > devices_info[ndevs-1].max_avail)
+			stripe_size = devices_info[ndevs-1].max_avail;
+	}
+
 	do_div(stripe_size, dev_stripes);
 
 	/* align to BTRFS_STRIPE_LEN */