Squashfs: add compression options support to xz decompressor

Pass the dictionary size used to compress datablocks.  Using a
dictionary size less than the block size saves memory overhead, in many
cases without adversely affecting compression ratio.
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>

fs/squashfs/xz_wrapper.c

@@ -26,6 +26,7 @@
 #include <linux/buffer_head.h>
 #include <linux/slab.h>
 #include <linux/xz.h>
+#include <linux/bitops.h>
 
 #include "squashfs_fs.h"
 #include "squashfs_fs_sb.h"
@@ -38,25 +39,57 @@ struct squashfs_xz {
 	struct xz_buf buf;
 };
 
+struct comp_opts {
+	__le32 dictionary_size;
+	__le32 flags;
+};
+
 static void *squashfs_xz_init(struct squashfs_sb_info *msblk, void *buff,
 	int len)
 {
-	int block_size = max_t(int, msblk->block_size, SQUASHFS_METADATA_SIZE);
+	struct comp_opts *comp_opts = buff;
+	struct squashfs_xz *stream;
+	int dict_size = msblk->block_size;
+	int err, n;
+
+	if (comp_opts) {
+		/* check compressor options are the expected length */
+		if (len < sizeof(*comp_opts)) {
+			err = -EIO;
+			goto failed;
+		}
 
-	struct squashfs_xz *stream = kmalloc(sizeof(*stream), GFP_KERNEL);
-	if (stream == NULL)
+		dict_size = le32_to_cpu(comp_opts->dictionary_size);
+
+		/* the dictionary size should be 2^n or 2^n+2^(n+1) */
+		n = ffs(dict_size) - 1;
+		if (dict_size != (1 << n) && dict_size != (1 << n) +
+						(1 << (n + 1))) {
+			err = -EIO;
+			goto failed;
+		}
+	}
+
+	dict_size = max_t(int, dict_size, SQUASHFS_METADATA_SIZE);
+
+	stream = kmalloc(sizeof(*stream), GFP_KERNEL);
+	if (stream == NULL) {
+		err = -ENOMEM;
 		goto failed;
+	}
 
-	stream->state = xz_dec_init(XZ_PREALLOC, block_size);
-	if (stream->state == NULL)
+	stream->state = xz_dec_init(XZ_PREALLOC, dict_size);
+	if (stream->state == NULL) {
+		kfree(stream);
+		err = -ENOMEM;
 		goto failed;
+	}
 
 	return stream;
 
 failed:
-	ERROR("Failed to allocate xz workspace\n");
-	kfree(stream);
-	return ERR_PTR(-ENOMEM);
+	ERROR("Failed to initialise xz decompressor\n");
+	return ERR_PTR(err);
 }