btrfs: reduce compressed_bio members' types

Several members of compressed_bio are of type that's unnecessarily big
for the values that they'd hold:

- the size of the uncompressed and compressed data is 128K now, we can
  keep is as int
- same for number of pages
- the compress type fits to a byte
- the errors is 0/1

The size of the unpatched structure is 80 bytes with several holes.
Reordering nr_pages next to the pages the hole after pending_bios is
filled and the resulting size is 56 bytes. This keeps the csums array
aligned to 8 bytes, which is nice. Further size optimizations may be
possible but right now it looks good to me:

struct compressed_bio {
        refcount_t                 pending_bios;         /*     0     4 */
        unsigned int               nr_pages;             /*     4     4 */
        struct page * *            compressed_pages;     /*     8     8 */
        struct inode *             inode;                /*    16     8 */
        u64                        start;                /*    24     8 */
        unsigned int               len;                  /*    32     4 */
        unsigned int               compressed_len;       /*    36     4 */
        u8                         compress_type;        /*    40     1 */
        u8                         errors;               /*    41     1 */

        /* XXX 2 bytes hole, try to pack */

        int                        mirror_num;           /*    44     4 */
        struct bio *               orig_bio;             /*    48     8 */
        u8                         sums[];               /*    56     0 */

        /* size: 56, cachelines: 1, members: 12 */
        /* sum members: 54, holes: 1, sum holes: 2 */
        /* last cacheline: 56 bytes */
};
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -507,7 +507,7 @@ blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
 		}
 		if (bytes_left < PAGE_SIZE) {
 			btrfs_info(fs_info,
-					"bytes left %lu compress len %lu nr %lu",
+					"bytes left %lu compress len %u nr %u",
 			       bytes_left, cb->compressed_len, cb->nr_pages);
 		}
 		bytes_left -= PAGE_SIZE;

fs/btrfs/compression.h

@@ -31,6 +31,9 @@ struct compressed_bio {
 	/* number of bios pending for this compressed extent */
 	refcount_t pending_bios;
 
+	/* Number of compressed pages in the array */
+	unsigned int nr_pages;
+
 	/* the pages with the compressed data on them */
 	struct page **compressed_pages;
 
@@ -40,20 +43,17 @@ struct compressed_bio {
 	/* starting offset in the inode for our pages */
 	u64 start;
 
-	/* number of bytes in the inode we're working on */
-	unsigned long len;
-
-	/* number of bytes on disk */
-	unsigned long compressed_len;
+	/* Number of bytes in the inode we're working on */
+	unsigned int len;
 
-	/* the compression algorithm for this bio */
-	int compress_type;
+	/* Number of bytes on disk */
+	unsigned int compressed_len;
 
-	/* number of compressed pages in the array */
-	unsigned long nr_pages;
+	/* The compression algorithm for this bio */
+	u8 compress_type;
 
 	/* IO errors */
-	int errors;
+	u8 errors;
 	int mirror_num;
 
 	/* for reads, this is the bio we are copying the data into */