[RFC][PATCH 3/3] ext4: add dio overwrite nolock

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From: Zheng Liu <wenqing.lz@xxxxxxxxxx>

Aligned and overwrite direct IO can be parallelized.  In ext4_file_dio_write,
we first check whether these conditions are satisfied or not.  If so, we unlock
the i_mutex and acquire i_data_sem directly.  Meanwhile iocb->private is set to
indicate that this is a overwrite dio, and it will be processed in
ext4_ext_direct_IO.

Signed-off-by: Zheng Liu <wenqing.lz@xxxxxxxxxx>
---
 fs/ext4/file.c |  140 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 137 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index e5d6be3..8a5f713 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -100,9 +100,21 @@ static ssize_t
 ext4_file_dio_write(struct kiocb *iocb, const struct iovec *iov,
 		    unsigned long nr_segs, loff_t pos)
 {
-	struct inode *inode = iocb->ki_filp->f_path.dentry->d_inode;
-	int unaligned_aio = 0;
+	struct file *file = iocb->ki_filp;
+	struct address_space * mapping = file->f_mapping;
+	struct inode *inode = file->f_path.dentry->d_inode;
+	struct blk_plug plug;
 	ssize_t ret;
+	ssize_t written, written_buffered;
+	size_t length = iov_length(iov, nr_segs);
+	size_t ocount;		/* original count */
+	size_t count;		/* after file limit checks */
+	int unaligned_aio = 0;
+	int overwrite = 0;
+	loff_t *ppos = &iocb->ki_pos;
+	loff_t endbyte;
+
+	BUG_ON(iocb->ki_pos != pos);
 
 	if (!is_sync_kiocb(iocb))
 		unaligned_aio = ext4_unaligned_aio(inode, iov, nr_segs, pos);
@@ -121,7 +133,129 @@ ext4_file_dio_write(struct kiocb *iocb, const struct iovec *iov,
 		ext4_aiodio_wait(inode);
 	}
 
-	ret = generic_file_aio_write(iocb, iov, nr_segs, pos);
+	mutex_lock(&inode->i_mutex);
+	blk_start_plug(&plug);
+
+	ocount = 0;
+	ret = generic_segment_checks(iov, &nr_segs, &ocount, VERIFY_READ);
+	if (ret)
+		goto unlock_out;
+
+	count = ocount;
+	pos = *ppos;
+
+	vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
+
+	/* We can write back this queue in page reclaim */
+	current->backing_dev_info = mapping->backing_dev_info;
+	written = 0;
+
+	ret = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
+	if (ret)
+		goto out;
+
+	if (count == 0)
+		goto out;
+
+	ret = file_remove_suid(file);
+	if (ret)
+		goto out;
+
+	file_update_time(file);
+
+	iocb->private = NULL;
+
+	if (!unaligned_aio && !file->f_mapping->nrpages &&
+	    pos + length < i_size_read(inode) &&
+	    ext4_should_dioread_nolock(inode)) {
+		struct ext4_map_blocks map;
+		unsigned int blkbits = inode->i_blkbits;
+		int err;
+		int len;
+
+		map.m_lblk = pos >> blkbits;
+		map.m_len = (EXT4_BLOCK_ALIGN(pos + length, blkbits) >> blkbits)
+			- map.m_lblk;
+		len = map.m_len;
+
+		err = ext4_map_blocks(NULL, inode, &map, 0);
+		if (err == len && (!map.m_flags ||
+		    map.m_flags & EXT4_MAP_MAPPED)) {
+			overwrite = 1;
+			iocb->private = &overwrite;
+			mutex_unlock(&inode->i_mutex);
+			down_read(&EXT4_I(inode)->i_data_sem);
+		}
+	}
+
+	if (file->f_mapping->nrpages && overwrite) {
+		overwrite = 0;
+		up_read(&EXT4_I(inode)->i_data_sem);
+		mutex_lock(&inode->i_mutex);
+	}
+
+	written = generic_file_direct_write(iocb, iov, &nr_segs, pos,
+						ppos, count, ocount);
+	if (written < 0 || written == count)
+		goto out;
+	/*
+	 * direct-io write to a hole: fall through to buffered I/O
+	 * for completing the rest of the request.
+	 */
+	pos += written;
+	count -= written;
+	written_buffered = generic_file_buffered_write(iocb, iov,
+					nr_segs, pos, ppos, count,
+					written);
+	/*
+	 * If generic_file_buffered_write() retuned a synchronous error
+	 * then we want to return the number of bytes which were
+	 * direct-written, or the error code if that was zero.  Note
+	 * that this differs from normal direct-io semantics, which
+	 * will return -EFOO even if some bytes were written.
+	 */
+	if (written_buffered < 0) {
+		ret = written_buffered;
+		goto out;
+	}
+
+	/*
+	 * We need to ensure that the page cache pages are written to
+	 * disk and invalidated to preserve the expected O_DIRECT
+	 * semantics.
+	 */
+	endbyte = pos + written_buffered - written - 1;
+	ret = filemap_write_and_wait_range(file->f_mapping, pos, endbyte);
+	if (ret == 0) {
+		written = written_buffered;
+		invalidate_mapping_pages(mapping,
+					 pos >> PAGE_CACHE_SHIFT,
+					 endbyte >> PAGE_CACHE_SHIFT);
+	} else {
+		/*
+		 * We don't know how much we wrote, so just return
+		 * the number of bytes which were direct-written
+		 */
+	}
+
+out:
+	current->backing_dev_info = NULL;
+	ret = written ? written : ret;
+
+unlock_out:
+	if (overwrite)
+		up_read(&EXT4_I(inode)->i_data_sem);
+	else
+		mutex_unlock(&inode->i_mutex);
+
+	if (ret > 0 || ret == -EIOCBQUEUED) {
+		ssize_t err;
+
+		err = generic_write_sync(file, pos, ret);
+		if (err < 0 && ret > 0)
+			ret = err;
+	}
+	blk_finish_plug(&plug);
 
 	if (unaligned_aio)
 		mutex_unlock(ext4_aio_mutex(inode));
-- 
1.7.1

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