[PATCH] change buffer_head.b_size to size_t

Increase the size of the buffer_head b_size field (only) for 64 bit platforms.
Update some old and moldy comments in and around the structure as well.

The b_size increase allows us to perform larger mappings and allocations for
large I/O requests from userspace, which tie in with other changes allowing
the get_block_t() interface to map multiple blocks at once.

Signed-off-by: Nathan Scott <nathans@sgi.com>
Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 11 insertions, 8 deletions

diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index da917ed096a3..464f068f8b16 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -46,25 +46,28 @@ struct address_space;
 typedef void (bh_end_io_t)(struct buffer_head *bh, int uptodate);
 
 /*
- * Keep related fields in common cachelines.  The most commonly accessed
- * field (b_state) goes at the start so the compiler does not generate
- * indexed addressing for it.
+ * Historically, a buffer_head was used to map a single block
+ * within a page, and of course as the unit of I/O through the
+ * filesystem and block layers.  Nowadays the basic I/O unit
+ * is the bio, and buffer_heads are used for extracting block
+ * mappings (via a get_block_t call), for tracking state within
+ * a page (via a page_mapping) and for wrapping bio submission
+ * for backward compatibility reasons (e.g. submit_bh).
  */
 struct buffer_head {
-        /* First cache line: */
         unsigned long b_state;          /* buffer state bitmap (see above) */
         struct buffer_head *b_this_page;/* circular list of page's buffers */
         struct page *b_page;            /* the page this bh is mapped to */
-        atomic_t b_count;               /* users using this block */
-        u32 b_size;                     /* block size */
 
-        sector_t b_blocknr;             /* block number */
-        char *b_data;                   /* pointer to data block */
+        sector_t b_blocknr;             /* start block number */
+        size_t b_size;                  /* size of mapping */
+        char *b_data;                   /* pointer to data within the page */
 
         struct block_device *b_bdev;
         bh_end_io_t *b_end_io;          /* I/O completion */
         void *b_private;                /* reserved for b_end_io */
         struct list_head b_assoc_buffers; /* associated with another mapping */
+        atomic_t b_count;               /* users using this buffer_head */
 };
 
 /*