linux-ext4 - [RFC PATCH 1/1] Remove buffered failover for ext4 and block fops direct writes.

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Date: Wed,  1 May 2024 16:15:33 -0700
From: Jeremy Bongio <bongiojp@...il.com>
To: Ted Tso <tytso@....edu>
Cc: linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org,
	linux-block@...r.kernel.org,
	Jeremy Bongio <jbongio@...gle.com>
Subject: [RFC PATCH 1/1] Remove buffered failover for ext4 and block fops direct writes.

From: Jeremy Bongio <jbongio@...gle.com>

ext4 and block fops would both failover to syncronous, buffered writes if
the direct IO results in a short write where only a portion of the request
was completed.

This patch changes the behavior to simply return the number of bytes
written if the direct write is short.
---
 block/fops.c   |  3 ---
 fs/ext4/file.c | 27 ---------------------------
 2 files changed, 30 deletions(-)

diff --git a/block/fops.c b/block/fops.c
index 0cf8cf72cdfa..d32574ba9d71 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -704,9 +704,6 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 
 	if (iocb->ki_flags & IOCB_DIRECT) {
 		ret = blkdev_direct_write(iocb, from);
-		if (ret >= 0 && iov_iter_count(from))
-			ret = direct_write_fallback(iocb, from, ret,
-					blkdev_buffered_write(iocb, from));
 	} else {
 		ret = blkdev_buffered_write(iocb, from);
 	}
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 54d6ff22585c..d0760452a11f 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -595,32 +595,6 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	else
 		inode_unlock(inode);
 
-	if (ret >= 0 && iov_iter_count(from)) {
-		ssize_t err;
-		loff_t endbyte;
-
-		offset = iocb->ki_pos;
-		err = ext4_buffered_write_iter(iocb, from);
-		if (err < 0)
-			return err;
-
-		/*
-		 * We need to ensure that the pages within the page cache for
-		 * the range covered by this I/O are written to disk and
-		 * invalidated. This is in attempt to preserve the expected
-		 * direct I/O semantics in the case we fallback to buffered I/O
-		 * to complete off the I/O request.
-		 */
-		ret += err;
-		endbyte = offset + err - 1;
-		err = filemap_write_and_wait_range(iocb->ki_filp->f_mapping,
-						   offset, endbyte);
-		if (!err)
-			invalidate_mapping_pages(iocb->ki_filp->f_mapping,
-						 offset >> PAGE_SHIFT,
-						 endbyte >> PAGE_SHIFT);
-	}
-
 	return ret;
 }
 
@@ -958,4 +932,3 @@ const struct inode_operations ext4_file_inode_operations = {
 	.fileattr_get	= ext4_fileattr_get,
 	.fileattr_set	= ext4_fileattr_set,
 };
-
-- 
2.44.0.769.g3c40516874-goog