btrfs: export compression buffer limits in a header

Move the buffer limit definitions out of compress_file_range.

Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.h

@@ -19,6 +19,21 @@
 #ifndef __BTRFS_COMPRESSION_
 #define __BTRFS_COMPRESSION_
 
+/*
+ * We want to make sure that amount of RAM required to uncompress an extent is
+ * reasonable, so we limit the total size in ram of a compressed extent to
+ * 128k.  This is a crucial number because it also controls how easily we can
+ * spread reads across cpus for decompression.
+ *
+ * We also want to make sure the amount of IO required to do a random read is
+ * reasonably small, so we limit the size of a compressed extent to 128k.
+ */
+
+/* Maximum length of compressed data stored on disk */
+#define BTRFS_MAX_COMPRESSED		(SZ_128K)
+/* Maximum size of data before compression */
+#define BTRFS_MAX_UNCOMPRESSED		(SZ_128K)
+
 void btrfs_init_compress(void);
 void btrfs_exit_compress(void);
 

fs/btrfs/inode.c

@@ -471,16 +471,6 @@ again:
 	   (start > 0 || end + 1 < BTRFS_I(inode)->disk_i_size))
 		goto cleanup_and_bail_uncompressed;
 
-	/* we want to make sure that amount of ram required to uncompress
-	 * an extent is reasonable, so we limit the total size in ram
-	 * of a compressed extent to 128k.  This is a crucial number
-	 * because it also controls how easily we can spread reads across
-	 * cpus for decompression.
-	 *
-	 * We also want to make sure the amount of IO required to do
-	 * a random read is reasonably small, so we limit the size of
-	 * a compressed extent to 128k.
-	 */
 	total_compressed = min(total_compressed, max_uncompressed);
 	num_bytes = ALIGN(end - start + 1, blocksize);
 	num_bytes = max(blocksize,  num_bytes);