btrfs: Fix threshold calculation for block groups smaller than 1GB

If a block group is smaller than 1GB, the extent entry threadhold
calculation will always set the threshold to 0.

So as free space gets fragmented, btrfs will switch to use bitmap
to manage free space, but then will never switch back to extents
due to this bug.

Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 60d684266959bbf8f96c14873e9a1f57e069a29e..42f4015988ecf1a2eeb44fe77e175a62b1282325 100644 (file)
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -1016,14 +1016,18 @@ static void recalculate_thresholds(struct btrfs_block_group_cache *block_group)
        u64 max_bytes;
        u64 bitmap_bytes;
        u64 extent_bytes;
+       u64 size = block_group->key.offset;
 
        /*
         * The goal is to keep the total amount of memory used per 1gb of space
         * at or below 32k, so we need to adjust how much memory we allow to be
         * used by extent based free space tracking
         */
-       max_bytes = MAX_CACHE_BYTES_PER_GIG *
-               (div64_u64(block_group->key.offset, 1024 * 1024 * 1024));
+       if (size < 1024 * 1024 * 1024)
+               max_bytes = MAX_CACHE_BYTES_PER_GIG;
+       else
+               max_bytes = MAX_CACHE_BYTES_PER_GIG *
+                       div64_u64(size, 1024 * 1024 * 1024);
 
        /*
         * we want to account for 1 more bitmap than what we have so we can make