[PATCH] reiserfs v3 readpages support

Patch from Chris Mason <mason@suse.com>

The patch below is against 2.5.59, various forms have been floating
around for a while, and Andrew recently included this fixed version in
2.5.55-mm.  The end result is faster reads and writes for reiserfs.

This adds reiserfs support for readpages, along with a support func in
fs/mpage.c to deal with the reiserfs_get_block call sending back up to
date buffers with packed tails copied into them.

Most of the changes are to reiserfs_writepage, which still had many
2.4isms in the way it started io, dealt with errors and handled the bh
state bits.  I've also added an optimization so it only starts
transactions when we need to copy a packed tail into the btree or fill a
hole, instead of any time reiserfs_writepage hits an unmapped buffer.

fs/mpage.c

@@ -116,6 +116,49 @@ mpage_alloc(struct block_device *bdev,
 	return bio;
 }
 
+/*
+ * support function for mpage_readpages.  The fs supplied get_block might
+ * return an up to date buffer.  This is used to map that buffer into
+ * the page, which allows readpage to avoid triggering a duplicate call
+ * to get_block.
+ *
+ * The idea is to avoid adding buffers to pages that don't already have
+ * them.  So when the buffer is up to date and the page size == block size,
+ * this marks the page up to date instead of adding new buffers.
+ */
+static void 
+map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block) 
+{
+	struct inode *inode = page->mapping->host;
+	struct buffer_head *page_bh, *head;
+	int block = 0;
+
+	if (!page_has_buffers(page)) {
+		/*
+		 * don't make any buffers if there is only one buffer on
+		 * the page and the page just needs to be set up to date
+		 */
+		if (inode->i_blkbits == PAGE_CACHE_SHIFT && 
+		    buffer_uptodate(bh)) {
+			SetPageUptodate(page);    
+			return;
+		}
+		create_empty_buffers(page, 1 << inode->i_blkbits, 0);
+	}
+	head = page_buffers(page);
+	page_bh = head;
+	do {
+		if (block == page_block) {
+			page_bh->b_state = bh->b_state;
+			page_bh->b_bdev = bh->b_bdev;
+			page_bh->b_blocknr = bh->b_blocknr;
+			break;
+		}
+		page_bh = page_bh->b_this_page;
+		block++;
+	} while (page_bh != head);
+}
+
 /**
  * mpage_readpages - populate an address space with some pages, and
  *                       start reads against them.
@@ -186,6 +229,7 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 	block_in_file = page->index << (PAGE_CACHE_SHIFT - blkbits);
 	last_block = (inode->i_size + blocksize - 1) >> blkbits;
 
+	bh.b_page = page;
 	for (page_block = 0; page_block < blocks_per_page;
 				page_block++, block_in_file++) {
 		bh.b_state = 0;
@@ -201,6 +245,17 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 			continue;
 		}
 
+		/* some filesystems will copy data into the page during
+		 * the get_block call, in which case we don't want to
+		 * read it again.  map_buffer_to_page copies the data
+		 * we just collected from get_block into the page's buffers
+		 * so readpage doesn't have to repeat the get_block call
+		 */
+		if (buffer_uptodate(&bh)) {
+			map_buffer_to_page(page, &bh, page_block);
+			goto confused;
+		}
+	
 		if (first_hole != blocks_per_page)
 			goto confused;		/* hole -> non-hole */
 
@@ -256,7 +311,10 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 confused:
 	if (bio)
 		bio = mpage_bio_submit(READ, bio);
+	if (!PageUptodate(page))
 	        block_read_full_page(page, get_block);
+	else
+		unlock_page(page);
 	goto out;
 }
 
@@ -344,6 +402,7 @@ mpage_writepage(struct bio *bio, struct page *page, get_block_t get_block,
 	sector_t boundary_block = 0;
 	struct block_device *boundary_bdev = NULL;
 	int length;
+	struct buffer_head map_bh;
 
 	if (page_has_buffers(page)) {
 		struct buffer_head *head = page_buffers(page);
@@ -401,8 +460,8 @@ mpage_writepage(struct bio *bio, struct page *page, get_block_t get_block,
 	BUG_ON(!PageUptodate(page));
 	block_in_file = page->index << (PAGE_CACHE_SHIFT - blkbits);
 	last_block = (inode->i_size - 1) >> blkbits;
+	map_bh.b_page = page;
 	for (page_block = 0; page_block < blocks_per_page; ) {
-		struct buffer_head map_bh;
 
 		map_bh.b_state = 0;
 		if (get_block(inode, block_in_file, &map_bh, 1))