[PATCH] f2fs: optimize f2fs DIO overwrites

[Yohan Joung <jyh429@gmail.com>]

this is unnecessary when we know we are overwriting already allocated
blocks and the overhead of starting a transaction can be significant
especially for multithreaded workloads doing small writes.

Signed-off-by: Yohan Joung <yohan.joung@sk.com>
---
 fs/f2fs/data.c | 20 ++++++++++++++++++++
 fs/f2fs/f2fs.h |  1 +
 fs/f2fs/file.c |  5 ++++-
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 09437dbd1b42..728630037b74 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -4267,6 +4267,26 @@ static int f2fs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	return 0;
 }
 
+static int f2fs_iomap_overwrite_begin(struct inode *inode, loff_t offset,
+		loff_t length, unsigned flags, struct iomap *iomap,
+		struct iomap *srcmap)
+{
+	int ret;
+
+	/*
+	 * Even for writes we don't need to allocate blocks, so just pretend
+	 * we are reading to save overhead of starting a transaction.
+	 */
+	flags &= ~IOMAP_WRITE;
+	ret = f2fs_iomap_begin(inode, offset, length, flags, iomap, srcmap);
+	WARN_ON_ONCE(!ret && iomap->type != IOMAP_MAPPED);
+	return ret;
+}
+
 const struct iomap_ops f2fs_iomap_ops = {
 	.iomap_begin	= f2fs_iomap_begin,
 };
+
+const struct iomap_ops f2fs_iomap_overwrite_ops = {
+	.iomap_begin	= f2fs_iomap_overwrite_begin,
+};
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index c6cc2694f9ac..0511ab5ed42a 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -3936,6 +3936,7 @@ void f2fs_destroy_post_read_processing(void);
 int f2fs_init_post_read_wq(struct f2fs_sb_info *sbi);
 void f2fs_destroy_post_read_wq(struct f2fs_sb_info *sbi);
 extern const struct iomap_ops f2fs_iomap_ops;
+extern const struct iomap_ops f2fs_iomap_overwrite_ops;
 
 static inline struct page *f2fs_find_data_page(struct inode *inode,
 		pgoff_t index, pgoff_t *next_pgofs)
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 82b21baf5628..bb2fe6dac9b6 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -4985,6 +4985,7 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
 	const ssize_t count = iov_iter_count(from);
 	unsigned int dio_flags;
 	struct iomap_dio *dio;
+	const struct iomap_ops *iomap_ops = &f2fs_iomap_ops;
 	ssize_t ret;
 
 	trace_f2fs_direct_IO_enter(inode, iocb, count, WRITE);
@@ -5025,7 +5026,9 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
 	dio_flags = 0;
 	if (pos + count > inode->i_size)
 		dio_flags |= IOMAP_DIO_FORCE_WAIT;
-	dio = __iomap_dio_rw(iocb, from, &f2fs_iomap_ops,
+	else if (f2fs_overwrite_io(inode, pos, count))
+		iomap_ops = &f2fs_iomap_overwrite_ops;
+	dio = __iomap_dio_rw(iocb, from, iomap_ops,
 			     &f2fs_iomap_dio_write_ops, dio_flags, NULL, 0);
 	if (IS_ERR_OR_NULL(dio)) {
 		ret = PTR_ERR_OR_ZERO(dio);
-- 
2.25.1