btrfs: merge length input and output parameter in compress_pages

The length parameter is basically duplicated for input and output in the
top level caller of the compress_pages chain. We can simply use one
variable for that and reduce stack consumption. The compression
implementation will sink the parameter to a local variable so everything
works as before.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -911,27 +911,25 @@ static void free_workspaces(void)
 }
 
 /*
- * given an address space and start/len, compress the bytes.
+ * Given an address space and start and length, compress the bytes into @pages
+ * that are allocated on demand.
  *
- * pages are allocated to hold the compressed result and stored
- * in 'pages'
+ * @out_pages is used to return the number of pages allocated.  There
+ * may be pages allocated even if we return an error.
  *
- * out_pages is used to return the number of pages allocated.  There
- * may be pages allocated even if we return an error
- *
- * total_in is used to return the number of bytes actually read.  It
- * may be smaller then len if we had to exit early because we
+ * @total_in is used to return the number of bytes actually read.  It
+ * may be smaller than the input length if we had to exit early because we
  * ran out of room in the pages array or because we cross the
  * max_out threshold.
  *
- * total_out is used to return the total number of compressed bytes
+ * @total_out is an in/out parameter, must be set to the input length and will
+ * be also used to return the total number of compressed bytes
  *
- * max_out tells us the max number of bytes that we're allowed to
+ * @max_out tells us the max number of bytes that we're allowed to
  * stuff into pages
  */
 int btrfs_compress_pages(int type, struct address_space *mapping,
-			 u64 start, unsigned long len,
-			 struct page **pages,
+			 u64 start, struct page **pages,
 			 unsigned long nr_dest_pages,
 			 unsigned long *out_pages,
 			 unsigned long *total_in,
@@ -944,7 +942,7 @@ int btrfs_compress_pages(int type, struct address_space *mapping,
 	workspace = find_workspace(type);
 
 	ret = btrfs_compress_op[type-1]->compress_pages(workspace, mapping,
-						      start, len, pages,
+						      start, pages,
 						      nr_dest_pages, out_pages,
 						      total_in, total_out,
 						      max_out);

fs/btrfs/compression.h

@@ -23,8 +23,7 @@ void btrfs_init_compress(void);
 void btrfs_exit_compress(void);
 
 int btrfs_compress_pages(int type, struct address_space *mapping,
-			 u64 start, unsigned long len,
-			 struct page **pages,
+			 u64 start, struct page **pages,
 			 unsigned long nr_dest_pages,
 			 unsigned long *out_pages,
 			 unsigned long *total_in,
@@ -59,7 +58,7 @@ struct btrfs_compress_op {
 
 	int (*compress_pages)(struct list_head *workspace,
 			      struct address_space *mapping,
-			      u64 start, unsigned long len,
+			      u64 start,
 			      struct page **pages,
 			      unsigned long nr_dest_pages,
 			      unsigned long *out_pages,

fs/btrfs/inode.c

@@ -517,7 +517,7 @@ again:
 		redirty = 1;
 		ret = btrfs_compress_pages(compress_type,
 					   inode->i_mapping, start,
-					   total_compressed, pages,
+					   pages,
 					   nr_pages, &nr_pages_ret,
 					   &total_in,
 					   &total_compressed,

fs/btrfs/lzo.c

@@ -86,7 +86,7 @@ static inline size_t read_compress_length(const char *buf)
 
 static int lzo_compress_pages(struct list_head *ws,
 			      struct address_space *mapping,
-			      u64 start, unsigned long len,
+			      u64 start,
 			      struct page **pages,
 			      unsigned long nr_dest_pages,
 			      unsigned long *out_pages,
@@ -102,7 +102,7 @@ static int lzo_compress_pages(struct list_head *ws,
 	struct page *in_page = NULL;
 	struct page *out_page = NULL;
 	unsigned long bytes_left;
-
+	unsigned long len = *total_out;
 	size_t in_len;
 	size_t out_len;
 	char *buf;

fs/btrfs/zlib.c

@@ -73,7 +73,7 @@ fail:
 
 static int zlib_compress_pages(struct list_head *ws,
 			       struct address_space *mapping,
-			       u64 start, unsigned long len,
+			       u64 start,
 			       struct page **pages,
 			       unsigned long nr_dest_pages,
 			       unsigned long *out_pages,
@@ -89,6 +89,7 @@ static int zlib_compress_pages(struct list_head *ws,
 	struct page *in_page = NULL;
 	struct page *out_page = NULL;
 	unsigned long bytes_left;
+	unsigned long len = *total_out;
 
 	*out_pages = 0;
 	*total_out = 0;