ext4: rewrite punch hole to use ext4_ext_remove_space()

This commit rewrites ext4 punch hole implementation to use
ext4_ext_remove_space() instead of its home gown way of doing this via
ext4_ext_map_blocks(). There are several reasons for changing this.

Firstly it is quite non obvious that punching hole needs to
ext4_ext_map_blocks() to punch a hole, especially given that this
function should map blocks, not unmap it. It also required a lot of new
code in ext4_ext_map_blocks().

Secondly the design of it is not very effective. The reason is that we
are trying to punch out blocks in ext4_ext_punch_hole() in opposite
direction than in ext4_ext_rm_leaf() which causes the ext4_ext_rm_leaf()
to iterate through the whole tree from the end to the start to find the
requested extent for every extent we are going to punch out.

And finally the current implementation does not use the existing code,
but bring a lot of new code, which is IMO unnecessary since there
already is some infrastructure we can use. Specifically
ext4_ext_remove_space().

This commit changes ext4_ext_remove_space() to accept 'end' parameter so
we can not only truncate to the end of file, but also remove the space
in the middle of the file (punch a hole). Moreover, because the last
block to punch out, might be in the middle of the extent, we have to
split the extent at 'end + 1' so ext4_ext_rm_leaf() can easily either
remove the whole fist part of split extent, or change its size.

ext4_ext_remove_space() is then used to actually remove the space
(extents) from within the hole, instead of ext4_ext_map_blocks().

Note that this also fix the issue with punch hole, where we would forget
to remove empty index blocks from the extent tree, resulting in double
free block error and file system corruption. This is simply because we
now use different code path, where this problem does not exist.

This has been tested with fsx running for several days and xfstests,
plus xfstest #251 with '-o discard' run on the loop image (which
converts discard requestes into punch hole to the backing file). All of
it on 1K and 4K file system block size.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -44,6 +44,14 @@
 
 #include <trace/events/ext4.h>
 
+/*
+ * used by extent splitting.
+ */
+#define EXT4_EXT_MAY_ZEROOUT	0x1  /* safe to zeroout if split fails \
+					due to ENOSPC */
+#define EXT4_EXT_MARK_UNINIT1	0x2  /* mark first half uninitialized */
+#define EXT4_EXT_MARK_UNINIT2	0x4  /* mark second half uninitialized */
+
 static int ext4_split_extent(handle_t *handle,
 				struct inode *inode,
 				struct ext4_ext_path *path,
@@ -51,6 +59,13 @@ static int ext4_split_extent(handle_t *handle,
 				int split_flag,
 				int flags);
 
+static int ext4_split_extent_at(handle_t *handle,
+			     struct inode *inode,
+			     struct ext4_ext_path *path,
+			     ext4_lblk_t split,
+			     int split_flag,
+			     int flags);
+
 static int ext4_ext_truncate_extend_restart(handle_t *handle,
 					    struct inode *inode,
 					    int needed)
@@ -2310,7 +2325,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
 	struct ext4_extent *ex;
 
 	/* the header must be checked already in ext4_ext_remove_space() */
-	ext_debug("truncate since %u in leaf\n", start);
+	ext_debug("truncate since %u in leaf to %u\n", start, end);
 	if (!path[depth].p_hdr)
 		path[depth].p_hdr = ext_block_hdr(path[depth].p_bh);
 	eh = path[depth].p_hdr;
@@ -2345,7 +2360,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
 		ext_debug("  border %u:%u\n", a, b);
 
 		/* If this extent is beyond the end of the hole, skip it */
-		if (end <= ex_ee_block) {
+		if (end < ex_ee_block) {
 			ex--;
 			ex_ee_block = le32_to_cpu(ex->ee_block);
 			ex_ee_len = ext4_ext_get_actual_len(ex);
@@ -2484,7 +2499,8 @@ ext4_ext_more_to_rm(struct ext4_ext_path *path)
 	return 1;
 }
 
-static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start)
+static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
+				 ext4_lblk_t end)
 {
 	struct super_block *sb = inode->i_sb;
 	int depth = ext_depth(inode);
@@ -2493,7 +2509,7 @@ static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start)
 	handle_t *handle;
 	int i, err;
 
-	ext_debug("truncate since %u\n", start);
+	ext_debug("truncate since %u to %u\n", start, end);
 
 	/* probably first extent we're gonna free will be last in block */
 	handle = ext4_journal_start(inode, depth + 1);
@@ -2505,6 +2521,61 @@ again:
 
 	trace_ext4_ext_remove_space(inode, start, depth);
 
+	/*
+	 * Check if we are removing extents inside the extent tree. If that
+	 * is the case, we are going to punch a hole inside the extent tree
+	 * so we have to check whether we need to split the extent covering
+	 * the last block to remove so we can easily remove the part of it
+	 * in ext4_ext_rm_leaf().
+	 */
+	if (end < EXT_MAX_BLOCKS - 1) {
+		struct ext4_extent *ex;
+		ext4_lblk_t ee_block;
+
+		/* find extent for this block */
+		path = ext4_ext_find_extent(inode, end, NULL);
+		if (IS_ERR(path)) {
+			ext4_journal_stop(handle);
+			return PTR_ERR(path);
+		}
+		depth = ext_depth(inode);
+		ex = path[depth].p_ext;
+		if (!ex)
+			goto cont;
+
+		ee_block = le32_to_cpu(ex->ee_block);
+
+		/*
+		 * See if the last block is inside the extent, if so split
+		 * the extent at 'end' block so we can easily remove the
+		 * tail of the first part of the split extent in
+		 * ext4_ext_rm_leaf().
+		 */
+		if (end >= ee_block &&
+		    end < ee_block + ext4_ext_get_actual_len(ex) - 1) {
+			int split_flag = 0;
+
+			if (ext4_ext_is_uninitialized(ex))
+				split_flag = EXT4_EXT_MARK_UNINIT1 |
+					     EXT4_EXT_MARK_UNINIT2;
+
+			/*
+			 * Split the extent in two so that 'end' is the last
+			 * block in the first new extent
+			 */
+			err = ext4_split_extent_at(handle, inode, path,
+						end + 1, split_flag,
+						EXT4_GET_BLOCKS_PRE_IO |
+						EXT4_GET_BLOCKS_PUNCH_OUT_EXT);
+
+			if (err < 0)
+				goto out;
+		}
+		ext4_ext_drop_refs(path);
+		kfree(path);
+	}
+cont:
+
 	/*
 	 * We start scanning from right side, freeing all the blocks
 	 * after i_size and walking into the tree depth-wise.
@@ -2517,6 +2588,7 @@ again:
 	}
 	path[0].p_depth = depth;
 	path[0].p_hdr = ext_inode_hdr(inode);
+
 	if (ext4_ext_check(inode, path[0].p_hdr, depth)) {
 		err = -EIO;
 		goto out;
@@ -2528,7 +2600,7 @@ again:
 			/* this is leaf block */
 			err = ext4_ext_rm_leaf(handle, inode, path,
 					       &partial_cluster, start,
-					       EXT_MAX_BLOCKS - 1);
+					       end);
 			/* root level has p_bh == NULL, brelse() eats this */
 			brelse(path[i].p_bh);
 			path[i].p_bh = NULL;
@@ -2710,14 +2782,6 @@ static int ext4_ext_zeroout(struct inode *inode, struct ext4_extent *ex)
 	return ret;
 }
 
-/*
- * used by extent splitting.
- */
-#define EXT4_EXT_MAY_ZEROOUT	0x1  /* safe to zeroout if split fails \
-					due to ENOSPC */
-#define EXT4_EXT_MARK_UNINIT1	0x2  /* mark first half uninitialized */
-#define EXT4_EXT_MARK_UNINIT2	0x4  /* mark second half uninitialized */
-
 /*
  * ext4_split_extent_at() splits an extent at given block.
  *
@@ -4230,7 +4294,7 @@ void ext4_ext_truncate(struct inode *inode)
 
 	last_block = (inode->i_size + sb->s_blocksize - 1)
 			>> EXT4_BLOCK_SIZE_BITS(sb);
-	err = ext4_ext_remove_space(inode, last_block);
+	err = ext4_ext_remove_space(inode, last_block, EXT_MAX_BLOCKS - 1);
 
 	/* In a multi-transaction truncate, we only make the final
 	 * transaction synchronous.
@@ -4707,14 +4771,12 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
 {
 	struct inode *inode = file->f_path.dentry->d_inode;
 	struct super_block *sb = inode->i_sb;
-	struct ext4_ext_cache cache_ex;
-	ext4_lblk_t first_block, last_block, num_blocks, iblock, max_blocks;
+	ext4_lblk_t first_block, stop_block;
 	struct address_space *mapping = inode->i_mapping;
-	struct ext4_map_blocks map;
 	handle_t *handle;
 	loff_t first_page, last_page, page_len;
 	loff_t first_page_offset, last_page_offset;
-	int ret, credits, blocks_released, err = 0;
+	int credits, err = 0;
 
 	/* No need to punch hole beyond i_size */
 	if (offset >= inode->i_size)
@@ -4730,10 +4792,6 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
 		   offset;
 	}
 
-	first_block = (offset + sb->s_blocksize - 1) >>
-		EXT4_BLOCK_SIZE_BITS(sb);
-	last_block = (offset + length) >> EXT4_BLOCK_SIZE_BITS(sb);
-
 	first_page = (offset + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
 	last_page = (offset + length) >> PAGE_CACHE_SHIFT;
 
@@ -4812,7 +4870,6 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
 		}
 	}
 
-
 	/*
 	 * If i_size is contained in the last page, we need to
 	 * unmap and zero the partial page after i_size
@@ -4832,73 +4889,22 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
 		}
 	}
 
+	first_block = (offset + sb->s_blocksize - 1) >>
+		EXT4_BLOCK_SIZE_BITS(sb);
+	stop_block = (offset + length) >> EXT4_BLOCK_SIZE_BITS(sb);
+
 	/* If there are no blocks to remove, return now */
-	if (first_block >= last_block)
+	if (first_block >= stop_block)
 		goto out;
 
 	down_write(&EXT4_I(inode)->i_data_sem);
 	ext4_ext_invalidate_cache(inode);
 	ext4_discard_preallocations(inode);
 
-	/*
-	 * Loop over all the blocks and identify blocks
-	 * that need to be punched out
-	 */
-	iblock = first_block;
-	blocks_released = 0;
-	while (iblock < last_block) {
-		max_blocks = last_block - iblock;
-		num_blocks = 1;
-		memset(&map, 0, sizeof(map));
-		map.m_lblk = iblock;
-		map.m_len = max_blocks;
-		ret = ext4_ext_map_blocks(handle, inode, &map,
-			EXT4_GET_BLOCKS_PUNCH_OUT_EXT);
-
-		if (ret > 0) {
-			blocks_released += ret;
-			num_blocks = ret;
-		} else if (ret == 0) {
-			/*
-			 * If map blocks could not find the block,
-			 * then it is in a hole.  If the hole was
-			 * not already cached, then map blocks should
-			 * put it in the cache.  So we can get the hole
-			 * out of the cache
-			 */
-			memset(&cache_ex, 0, sizeof(cache_ex));
-			if ((ext4_ext_check_cache(inode, iblock, &cache_ex)) &&
-				!cache_ex.ec_start) {
+	err = ext4_ext_remove_space(inode, first_block, stop_block - 1);
 
-				/* The hole is cached */
-				num_blocks = cache_ex.ec_block +
-				cache_ex.ec_len - iblock;
-
-			} else {
-				/* The block could not be identified */
-				err = -EIO;
-				break;
-			}
-		} else {
-			/* Map blocks error */
-			err = ret;
-			break;
-		}
-
-		if (num_blocks == 0) {
-			/* This condition should never happen */
-			ext_debug("Block lookup failed");
-			err = -EIO;
-			break;
-		}
-
-		iblock += num_blocks;
-	}
-
-	if (blocks_released > 0) {
-		ext4_ext_invalidate_cache(inode);
-		ext4_discard_preallocations(inode);
-	}
+	ext4_ext_invalidate_cache(inode);
+	ext4_discard_preallocations(inode);
 
 	if (IS_SYNC(inode))
 		ext4_handle_sync(handle);