[PATCH] ext4: correctly calculate number of blocks for fiemap

[Surbhi Palande]

The following cherry-picked patch fixes the calculation of extents in fiemap.
Without this patch a user of fiemap could be misled into thinking that there
are no extents in a file when actually there are. This patch is necessary for
code such as ureadahead which uses fiemap to find out if the file extents have
increased.

The commit id in the upstream code is: aca92ff6f57c000d1b4523e383c8bd6b8269b8b1
This patch fixes bug 474597 on LP. Do consider merging this in Lucid.

>From dfaa33f0502a8bf9dff0e596783a40ce330c7a9b Mon Sep 17 00:00:00 2001
From: Leonard Michlmayr <leonard.michlmayr at gmail.com>
Date: Thu, 4 Mar 2010 17:07:28 -0500
Subject: [PATCH] ext4: correctly calculate number of blocks for fiemap

ext4_fiemap() rounds the length of the requested range down to
blocksize, which is is not the true number of blocks that cover the
requested region.  This problem is especially impressive if the user
requests only the first byte of a file: not a single extent will be
reported.

We fix this by calculating the last block of the region and then
subtract to find the number of blocks in the extents.

Signed-off-by: Leonard Michlmayr <leonard.michlmayr at gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso at mit.edu>
(cherry picked from commit aca92ff6f57c000d1b4523e383c8bd6b8269b8b1 upstream)

BugLink: http://launchpad.net/bugs/474597
Signed-off-by: Surbhi Palande <surbhi.palande at canonical.com>
---
 fs/ext4/extents.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 8b8bae4..f199d20 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3711,7 +3711,6 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		__u64 start, __u64 len)
 {
 	ext4_lblk_t start_blk;
-	ext4_lblk_t len_blks;
 	int error = 0;
 
 	/* fallback to generic here if not in extents fmt */
@@ -3725,8 +3724,14 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	if (fieinfo->fi_flags & FIEMAP_FLAG_XATTR) {
 		error = ext4_xattr_fiemap(inode, fieinfo);
 	} else {
+		ext4_lblk_t len_blks;
+		__u64 last_blk;
+
 		start_blk = start >> inode->i_sb->s_blocksize_bits;
-		len_blks = len >> inode->i_sb->s_blocksize_bits;
+		last_blk = (start + len - 1) >> inode->i_sb->s_blocksize_bits;
+		if (last_blk >= EXT_MAX_BLOCK)
+			last_blk = EXT_MAX_BLOCK-1;
+		len_blks = ((ext4_lblk_t) last_blk) - start_blk + 1;
 
 		/*
 		 * Walk the extent tree gathering extent information.
-- 
1.6.3.3