[PATCH] ext4: Allocate entire range in zero range

[Lukas Czerner <lczerner@redhat.com>]

Currently there is a bug in zero range code which causes zero range
calls to only allocate block aligned portion of the range, while
ignoring the rest in some cases.

In some cases, namely if the end of the range is past isize, we do
attempt to preallocate the last nonaligned block. However this might
cause kernel to BUG() in some carefully designed zero range requests on
setups where page size > block size.

Fix this problem by first preallocating the entire range, including the
nonaligned edges and converting the written extents to unwritten in the
next step. This approach will also give us the advantage of having the
range to be as linearly contiguous as possible.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
 fs/ext4/extents.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index bed4308..aa52242 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4803,12 +4803,6 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 	else
 		max_blocks -= lblk;
 
-	flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT |
-		EXT4_GET_BLOCKS_CONVERT_UNWRITTEN |
-		EXT4_EX_NOCACHE;
-	if (mode & FALLOC_FL_KEEP_SIZE)
-		flags |= EXT4_GET_BLOCKS_KEEP_SIZE;
-
 	mutex_lock(&inode->i_mutex);
 
 	/*
@@ -4825,15 +4819,28 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 		ret = inode_newsize_ok(inode, new_size);
 		if (ret)
 			goto out_mutex;
-		/*
-		 * If we have a partial block after EOF we have to allocate
-		 * the entire block.
-		 */
-		if (partial_end)
-			max_blocks += 1;
 	}
 
+	flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT;
+	if (mode & FALLOC_FL_KEEP_SIZE)
+		flags |= EXT4_GET_BLOCKS_KEEP_SIZE;
+
+	/* Preallocate the range including the unaligned edges */
+	if (partial_begin || partial_end) {
+		ret = ext4_alloc_file_blocks(file,
+				round_down(offset, 1 << blkbits) >> blkbits,
+				(round_up((offset + len), 1 << blkbits) -
+				 round_down(offset, 1 << blkbits)) >> blkbits,
+				new_size, flags, mode);
+		if (ret)
+			goto out_mutex;
+
+	}
+
+	/* Zero range excluding the unaligned edges */
 	if (max_blocks > 0) {
+		flags |= (EXT4_GET_BLOCKS_CONVERT_UNWRITTEN |
+			  EXT4_EX_NOCACHE);
 
 		/* Now release the pages and zero block aligned part of pages*/
 		truncate_pagecache_range(inode, start, end - 1);
-- 
1.8.3.1