Revert "btrfs: switch to iomap_dio_rw() for dio"

This reverts commit a43a67a.

This patch reverts the main part of switching direct io implementation
to iomap infrastructure. There's a problem in invalidate page that
couldn't be solved as regression in this development cycle.

The problem occurs when buffered and direct io are mixed, and the ranges
overlap. Although this is not recommended, filesystems implement
measures or fallbacks to make it somehow work. In this case, fallback to
buffered IO would be an option for btrfs (this already happens when
direct io is done on compressed data), but the change would be needed in
the iomap code, bringing new semantics to other filesystems.

Another problem arises when again the buffered and direct ios are mixed,
invalidation fails, then -EIO is set on the mapping and fsync will fail,
though there's no real error.

There have been discussions how to fix that, but revert seems to be the
least intrusive option.

Link: https://lore.kernel.org/linux-btrfs/20200528192103.xm45qoxqmkw7i5yl@fiona/
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/Kconfig

@@ -14,7 +14,6 @@ config BTRFS_FS
  select LZO_DECOMPRESS
  select ZSTD_COMPRESS
  select ZSTD_DECOMPRESS
- select FS_IOMAP
  select RAID6_PQ
  select XOR_BLOCKS
  select SRCU

fs/btrfs/ctree.h

@@ -2933,7 +2933,6 @@ int btrfs_writepage_cow_fixup(struct page *page, u64 start, u64 end);
 void btrfs_writepage_endio_finish_ordered(struct page *page, u64 start,
  u64 end, int uptodate);
 extern const struct dentry_operations btrfs_dentry_operations;
-ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter);
 
 /* ioctl.c */
 long btrfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg);

fs/btrfs/file.c

@@ -1819,7 +1819,7 @@ static ssize_t __btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
  loff_t endbyte;
  int err;
 
- written = btrfs_direct_IO(iocb, from);
+ written = generic_file_direct_write(iocb, from);
 
  if (written < 0 || !iov_iter_count(from))
  return written;
@@ -3476,26 +3476,9 @@ static int btrfs_file_open(struct inode *inode, struct file *filp)
  return generic_file_open(inode, filp);
 }
 
-static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
-{
- ssize_t ret = 0;
-
- if (iocb->ki_flags & IOCB_DIRECT) {
- struct inode *inode = file_inode(iocb->ki_filp);
-
- inode_lock_shared(inode);
- ret = btrfs_direct_IO(iocb, to);
- inode_unlock_shared(inode);
- if (ret < 0)
- return ret;
- }
-
- return generic_file_buffered_read(iocb, to, ret);
-}
-
 const struct file_operations btrfs_file_operations = {
  .llseek = btrfs_file_llseek,
- .read_iter = btrfs_file_read_iter,
+ .read_iter = generic_file_read_iter,
  .splice_read = generic_file_splice_read,
  .write_iter = btrfs_file_write_iter,
  .mmap = btrfs_file_mmap,