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_quota_off(struct super_block *sb, int type)
{
	struct inode *inode = sb_dqopt(sb)->files[type];
	handle_t *handle;

	/* Force all delayed allocation blocks to be allocated.
	 * Caller already holds s_umount sem */
	if (test_opt(sb, DELALLOC))
		sync_filesystem(sb);

	if (!inode)
		goto out;

	/* Update modification times of quota files when userspace can
	 * start looking at them */
	handle = ext4_journal_start(inode, EXT4_HT_QUOTA, 1);
	if (IS_ERR(handle))
		goto out;
	inode->i_mtime = inode->i_ctime = CURRENT_TIME;
	ext4_mark_inode_dirty(handle, inode);
	ext4_journal_stop(handle);

out:
	return dquot_quota_off(sb, type);
}

static int ext4_quota_off(struct super_block *sb, int type)
{
	struct inode *inode = sb_dqopt(sb)->files[type];
	handle_t *handle;

	/* Force all delayed allocation blocks to be allocated.
	 * Caller already holds s_umount sem */
	if (test_opt(sb, DELALLOC))
		sync_filesystem(sb);

	if (!inode)
		goto out;

	/* Update modification times of quota files when userspace can
	 * start looking at them */
	handle = ext4_journal_start(inode, EXT4_HT_QUOTA, 1);
	if (IS_ERR(handle))
		goto out;
	inode->i_mtime = inode->i_ctime = CURRENT_TIME;
	ext4_mark_inode_dirty(handle, inode);
	ext4_journal_stop(handle);

out:
	return dquot_quota_off(sb, type);
}
