[RFC 5/8] ext4: Adds direct-io atomic writes checks

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

This patch adds ext4 specific checks for supporting atomic writes
using fsawu (filesystem atomic write unit). We can enable this support
with either -
1. bigalloc on a 4k pagesize system or
2. bs < ps system with -b <BS>
3. filesystems with LBS (large block size) support (future)

Let's use generic_atomic_write_valid() helper for alignment
restrictions checking.

Co-developed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 fs/ext4/file.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 54d6ff22585c..8e309a9a0bd6 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -400,6 +400,21 @@ static const struct iomap_dio_ops ext4_dio_write_ops = {
 	.end_io = ext4_dio_write_end_io,
 };
 
+static bool ext4_dio_atomic_write_checks(struct kiocb *iocb,
+					 struct iov_iter *from)
+{
+	struct super_block *sb = file_inode(iocb->ki_filp)->i_sb;
+	loff_t pos = iocb->ki_pos;
+	unsigned int fsawu_min, fsawu_max;
+
+	if (!ext4_can_atomic_write_fsawu(sb))
+		return false;
+
+	ext4_atomic_write_fsawu(sb, &fsawu_min, &fsawu_max);
+
+	return generic_atomic_write_valid(pos, from, fsawu_min, fsawu_max);
+}
+
 /*
  * The intention here is to start with shared lock acquired then see if any
  * condition requires an exclusive inode lock. If yes, then we restart the
@@ -427,13 +442,19 @@ static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
 	loff_t offset;
 	size_t count;
 	ssize_t ret;
-	bool overwrite, unaligned_io;
+	bool overwrite, unaligned_io, atomic_write;
 
 restart:
 	ret = ext4_generic_write_checks(iocb, from);
 	if (ret <= 0)
 		goto out;
 
+	atomic_write = iocb->ki_flags & IOCB_ATOMIC;
+	if (atomic_write && !ext4_dio_atomic_write_checks(iocb, from)) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	offset = iocb->ki_pos;
 	count = ret;
 
@@ -576,8 +597,15 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		iomap_ops = &ext4_iomap_overwrite_ops;
 	ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
 			   dio_flags, NULL, 0);
-	if (ret == -ENOTBLK)
-		ret = 0;
+
+	/* Fallback to buffered-io for non-atomic DIO */
+	if (ret == -ENOTBLK) {
+		if (iocb->ki_flags & IOCB_ATOMIC)
+			ret = -EIO;
+		else
+			ret = 0;
+	}
+
 	if (extend) {
 		/*
 		 * We always perform extending DIO write synchronously so by
-- 
2.43.0