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>

1 files changed, 41 insertions, 8 deletions

diff --git a/fs/squashfs/xz_wrapper.c b/fs/squashfs/xz_wrapper.c
index 397adea72eb9..06d0d11b482a 100644
--- a/fs/squashfs/xz_wrapper.c
+++ b/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);
 }