Btrfs: fix the page that is beyond EOF

Steps to reproduce:
 # mkfs.btrfs <disk>
 # mount <disk> <mnt>
 # dd if=/dev/zero of=<mnt>/<file> bs=512 seek=5 count=8
 # fallocate -p -o 2048 -l 16384 <mnt>/<file>
 # dd if=/dev/zero of=<mnt>/<file> bs=4096 seek=3 count=8 conv=notrunc,nocreat
 # umount <mnt>
 # dmesg
 WARNING: at fs/btrfs/inode.c:7140 btrfs_destroy_inode+0x2eb/0x330

The reason is that we inputed a range which is beyond the end of the file. And
because the end of this range was not page-aligned, we had to truncate the last
page in this range, this operation is similar to a buffered file write. In other
words, we reserved enough space and clear the data which was in the hole range
on that page. But when we expanded that test file, write the data into the same
page, we forgot that we have reserved enough space for the buffered write of
that page because in most cases there is no page that is beyond the end of
the file. As a result, we reserved the space twice.

In fact, we needn't truncate the page if it is beyond the end of the file, just
release the allocated space in that range. Fix the above problem by this way.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/file.c

@@ -1859,9 +1859,9 @@ static int btrfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 	struct btrfs_path *path;
 	struct btrfs_block_rsv *rsv;
 	struct btrfs_trans_handle *trans;
-	u64 mask = BTRFS_I(inode)->root->sectorsize - 1;
-	u64 lockstart = (offset + mask) & ~mask;
-	u64 lockend = ((offset + len) & ~mask) - 1;
+	u64 lockstart = round_up(offset, BTRFS_I(inode)->root->sectorsize);
+	u64 lockend = round_down(offset + len,
+				 BTRFS_I(inode)->root->sectorsize) - 1;
 	u64 cur_offset = lockstart;
 	u64 min_size = btrfs_calc_trunc_metadata_size(root, 1);
 	u64 drop_end;
@@ -1896,10 +1896,12 @@ static int btrfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 	}
 
 	/* zero the front end of the last page */
-	ret = btrfs_truncate_page(inode, offset + len, 0, 1);
-	if (ret) {
-		mutex_unlock(&inode->i_mutex);
-		return ret;
+	if (offset + len < round_up(inode->i_size, PAGE_CACHE_SIZE)) {
+		ret = btrfs_truncate_page(inode, offset + len, 0, 1);
+		if (ret) {
+			mutex_unlock(&inode->i_mutex);
+			return ret;
+		}
 	}
 
 	if (lockend < lockstart) {