ext4: fix races between page faults and hole punching

Currently, page faults and hole punching are completely unsynchronized.
This can result in page fault faulting in a page into a range that we
are punching after truncate_pagecache_range() has been called and thus
we can end up with a page mapped to disk blocks that will be shortly
freed. Filesystem corruption will shortly follow. Note that the same
race is avoided for truncate by checking page fault offset against
i_size but there isn't similar mechanism available for punching holes.

Fix the problem by creating new rw semaphore i_mmap_sem in inode and
grab it for writing over truncate, hole punching, and other functions
removing blocks from extent tree and for read over page faults. We
cannot easily use i_data_sem for this since that ranks below transaction
start and we need something ranking above it so that it can be held over
the whole truncate / hole punching operation. Also remove various
workarounds we had in the code to reduce race window when page fault
could have created pages with stale mapping information.

Signed-off-by: Jan Kara <jack@suse.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
static int ext4_split_extent(handle_t *handle,
			      struct inode *inode,
			      struct ext4_ext_path **ppath,
			      struct ext4_map_blocks *map,
			      int split_flag,
			      int flags)
{
	struct ext4_ext_path *path = *ppath;
	ext4_lblk_t ee_block;
	struct ext4_extent *ex;
	unsigned int ee_len, depth;
	int err = 0;
	int unwritten;
	int split_flag1, flags1;
	int allocated = map->m_len;

	depth = ext_depth(inode);
	ex = path[depth].p_ext;
	ee_block = le32_to_cpu(ex->ee_block);
	ee_len = ext4_ext_get_actual_len(ex);
	unwritten = ext4_ext_is_unwritten(ex);

	if (map->m_lblk + map->m_len < ee_block + ee_len) {
		split_flag1 = split_flag & EXT4_EXT_MAY_ZEROOUT;
		flags1 = flags | EXT4_GET_BLOCKS_PRE_IO;
		if (unwritten)
			split_flag1 |= EXT4_EXT_MARK_UNWRIT1 |
				       EXT4_EXT_MARK_UNWRIT2;
		if (split_flag & EXT4_EXT_DATA_VALID2)
			split_flag1 |= EXT4_EXT_DATA_VALID1;
		err = ext4_split_extent_at(handle, inode, ppath,
				map->m_lblk + map->m_len, split_flag1, flags1);
		if (err)
			goto out;
	} else {
		allocated = ee_len - (map->m_lblk - ee_block);
	}
	/*
	 * Update path is required because previous ext4_split_extent_at() may
	 * result in split of original leaf or extent zeroout.
	 */
	path = ext4_find_extent(inode, map->m_lblk, ppath, 0);
	if (IS_ERR(path))
		return PTR_ERR(path);
	depth = ext_depth(inode);
	ex = path[depth].p_ext;
	if (!ex) {
		EXT4_ERROR_INODE(inode, "unexpected hole at %lu",
				 (unsigned long) map->m_lblk);
		return -EFSCORRUPTED;
	}
	unwritten = ext4_ext_is_unwritten(ex);
	split_flag1 = 0;

	if (map->m_lblk >= ee_block) {
		split_flag1 = split_flag & EXT4_EXT_DATA_VALID2;
		if (unwritten) {
			split_flag1 |= EXT4_EXT_MARK_UNWRIT1;
			split_flag1 |= split_flag & (EXT4_EXT_MAY_ZEROOUT |
						     EXT4_EXT_MARK_UNWRIT2);
		}
		err = ext4_split_extent_at(handle, inode, ppath,
				map->m_lblk, split_flag1, flags);
		if (err)
			goto out;
	}

	ext4_ext_show_leaf(inode, path);
out:
	return err ? err : allocated;
}

static int ext4_split_extent(handle_t *handle,
			      struct inode *inode,
			      struct ext4_ext_path **ppath,
			      struct ext4_map_blocks *map,
			      int split_flag,
			      int flags)
{
	struct ext4_ext_path *path = *ppath;
	ext4_lblk_t ee_block;
	struct ext4_extent *ex;
	unsigned int ee_len, depth;
	int err = 0;
	int unwritten;
	int split_flag1, flags1;
	int allocated = map->m_len;

	depth = ext_depth(inode);
	ex = path[depth].p_ext;
	ee_block = le32_to_cpu(ex->ee_block);
	ee_len = ext4_ext_get_actual_len(ex);
	unwritten = ext4_ext_is_unwritten(ex);

	if (map->m_lblk + map->m_len < ee_block + ee_len) {
		split_flag1 = split_flag & EXT4_EXT_MAY_ZEROOUT;
		flags1 = flags | EXT4_GET_BLOCKS_PRE_IO;
		if (unwritten)
			split_flag1 |= EXT4_EXT_MARK_UNWRIT1 |
				       EXT4_EXT_MARK_UNWRIT2;
		if (split_flag & EXT4_EXT_DATA_VALID2)
			split_flag1 |= EXT4_EXT_DATA_VALID1;
		err = ext4_split_extent_at(handle, inode, ppath,
				map->m_lblk + map->m_len, split_flag1, flags1);
		if (err)
			goto out;
	} else {
		allocated = ee_len - (map->m_lblk - ee_block);
	}
	/*
	 * Update path is required because previous ext4_split_extent_at() may
	 * result in split of original leaf or extent zeroout.
	 */
	path = ext4_find_extent(inode, map->m_lblk, ppath, 0);
	if (IS_ERR(path))
		return PTR_ERR(path);
	depth = ext_depth(inode);
	ex = path[depth].p_ext;
	if (!ex) {
		EXT4_ERROR_INODE(inode, "unexpected hole at %lu",
				 (unsigned long) map->m_lblk);
		return -EFSCORRUPTED;
	}
	unwritten = ext4_ext_is_unwritten(ex);
	split_flag1 = 0;

	if (map->m_lblk >= ee_block) {
		split_flag1 = split_flag & EXT4_EXT_DATA_VALID2;
		if (unwritten) {
			split_flag1 |= EXT4_EXT_MARK_UNWRIT1;
			split_flag1 |= split_flag & (EXT4_EXT_MAY_ZEROOUT |
						     EXT4_EXT_MARK_UNWRIT2);
		}
		err = ext4_split_extent_at(handle, inode, ppath,
				map->m_lblk, split_flag1, flags);
		if (err)
			goto out;
	}

	ext4_ext_show_leaf(inode, path);
out:
	return err ? err : allocated;
}
