[PATCH] do_mpage_readpage(): remove first_logical_block parameter

[Franck Bui-Huu <vagabon.xyz@gmail.com>]

From: Franck Bui-Huu <fbuihuu@gmail.com>

This patch makes this function more readable IMHO by getting rid of
its parameter 'first_logical_block'.

This parameter was previously used to remember the original block
number in the file (ie the page index) that 'map_bh' was pointing
at first. Indeed this was needed since 'map_bh->page' was modified
even if 'map_bh' maps the whole current page (IOW when get_block()
is not used). This had the bad effect to make the state of 'map_bh'
inconsistent too.

This patch changes 'map_bh->page' only when get_block() is called
hence removes the need of the 'first_logical_block' argument.

The value stored in 'logical_block_parameter' is now recalculated
each time do_mpage_readpage() but it shouldn't matter since the
operation is trivial.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
---

   Andrew,

 I made this when trying to understand this function. IMHO it
 makes the code more readable.

 I still don't know why 'map_bh->page' was setup every time
 do_mpage_readpage() is called, but I'm just discovering this
 code, so...

		Franck

 fs/mpage.c |   57 ++++++++++++++++++++++++++++-----------------------------
 1 files changed, 28 insertions(+), 29 deletions(-)

diff --git a/fs/mpage.c b/fs/mpage.c
index cf05ca7..da4d66f 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -168,7 +168,7 @@ map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
 static struct bio *
 do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 		sector_t *last_block_in_bio, struct buffer_head *map_bh,
-		unsigned long *first_logical_block, get_block_t get_block)
+		get_block_t get_block)
 {
 	struct inode *inode = page->mapping->host;
 	const unsigned blkbits = inode->i_blkbits;
@@ -184,7 +184,7 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 	int length;
 	int fully_mapped = 1;
 	unsigned nblocks;
-	unsigned relative_block;
+	unsigned i;
 
 	if (page_has_buffers(page))
 		goto confused;
@@ -199,40 +199,42 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 	/*
 	 * Map blocks using the result from the previous get_blocks call first.
 	 */
-	nblocks = map_bh->b_size >> blkbits;
-	if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
-			block_in_file < (*first_logical_block + nblocks)) {
-		unsigned map_offset = block_in_file - *first_logical_block;
-		unsigned last = nblocks - map_offset;
-
-		for (relative_block = 0; ; relative_block++) {
-			if (relative_block == last) {
-				clear_buffer_mapped(map_bh);
-				break;
+	if (buffer_mapped(map_bh)) {
+		sector_t map_bk = map_bh->b_page->index << \
+					(PAGE_CACHE_SHIFT - blkbits);
+
+		nblocks = map_bh->b_size >> blkbits;
+		if (block_in_file > map_bk && block_in_file < map_bk + nblocks) {
+			unsigned map_offset = block_in_file - map_bk;
+			sector_t map_blocknr = map_bh->b_blocknr;
+
+			for (i = map_offset; ; i++) {
+				if (i == nblocks) {
+					clear_buffer_mapped(map_bh);
+					break;
+				}
+				if (page_block == blocks_per_page)
+					break;
+				blocks[page_block] = map_blocknr + i;
+				page_block++;
+				block_in_file++;
 			}
-			if (page_block == blocks_per_page)
-				break;
-			blocks[page_block] = map_bh->b_blocknr + map_offset +
-						relative_block;
-			page_block++;
-			block_in_file++;
+			bdev = map_bh->b_bdev;
 		}
-		bdev = map_bh->b_bdev;
 	}
 
 	/*
 	 * Then do more get_blocks calls until we are done with this page.
 	 */
-	map_bh->b_page = page;
 	while (page_block < blocks_per_page) {
 		map_bh->b_state = 0;
 		map_bh->b_size = 0;
 
 		if (block_in_file < last_block) {
+			map_bh->b_page = page;
 			map_bh->b_size = (last_block-block_in_file) << blkbits;
 			if (get_block(inode, block_in_file, map_bh, 0))
 				goto confused;
-			*first_logical_block = block_in_file;
 		}
 
 		if (!buffer_mapped(map_bh)) {
@@ -254,7 +256,7 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 			map_buffer_to_page(page, map_bh, page_block);
 			goto confused;
 		}
-	
+
 		if (first_hole != blocks_per_page)
 			goto confused;		/* hole -> non-hole */
 
@@ -262,13 +264,13 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 		if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
 			goto confused;
 		nblocks = map_bh->b_size >> blkbits;
-		for (relative_block = 0; ; relative_block++) {
-			if (relative_block == nblocks) {
+		for (i = 0; ; i++) {
+			if (i == nblocks) {
 				clear_buffer_mapped(map_bh);
 				break;
 			} else if (page_block == blocks_per_page)
 				break;
-			blocks[page_block] = map_bh->b_blocknr+relative_block;
+			blocks[page_block] = map_bh->b_blocknr + i;
 			page_block++;
 			block_in_file++;
 		}
@@ -375,7 +377,6 @@ mpage_readpages(struct address_space *mapping, struct list_head *pages,
 	unsigned page_idx;
 	sector_t last_block_in_bio = 0;
 	struct buffer_head map_bh;
-	unsigned long first_logical_block = 0;
 
 	clear_buffer_mapped(&map_bh);
 	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
@@ -388,7 +389,6 @@ mpage_readpages(struct address_space *mapping, struct list_head *pages,
 			bio = do_mpage_readpage(bio, page,
 					nr_pages - page_idx,
 					&last_block_in_bio, &map_bh,
-					&first_logical_block,
 					get_block);
 		}
 		page_cache_release(page);
@@ -408,11 +408,10 @@ int mpage_readpage(struct page *page, get_block_t get_block)
 	struct bio *bio = NULL;
 	sector_t last_block_in_bio = 0;
 	struct buffer_head map_bh;
-	unsigned long first_logical_block = 0;
 
 	clear_buffer_mapped(&map_bh);
 	bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
-			&map_bh, &first_logical_block, get_block);
+			&map_bh, get_block);
 	if (bio)
 		mpage_bio_submit(READ, bio);
 	return 0;
-- 
1.6.0.2.GIT