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_valid_extent(struct inode *inode, struct ext4_extent *ext)
{
	ext4_fsblk_t block = ext4_ext_pblock(ext);
	int len = ext4_ext_get_actual_len(ext);
	ext4_lblk_t lblock = le32_to_cpu(ext->ee_block);
	ext4_lblk_t last = lblock + len - 1;

	if (len == 0 || lblock > last)
		return 0;
	return ext4_data_block_valid(EXT4_SB(inode->i_sb), block, len);
}

static int ext4_valid_extent(struct inode *inode, struct ext4_extent *ext)
{
	ext4_fsblk_t block = ext4_ext_pblock(ext);
	int len = ext4_ext_get_actual_len(ext);
	ext4_lblk_t lblock = le32_to_cpu(ext->ee_block);
	ext4_lblk_t last = lblock + len - 1;

	if (len == 0 || lblock > last)
		return 0;
	return ext4_data_block_valid(EXT4_SB(inode->i_sb), block, len);
}
