linux-kernel - [PATCH v6 02/13] iomap: comment on atomic write checks in iomap_dio_bio

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Message-Id: <20250313171310.1886394-3-john.g.garry@oracle.com>
Date: Thu, 13 Mar 2025 17:12:59 +0000
From: John Garry <john.g.garry@...cle.com>
To: brauner@...nel.org, djwong@...nel.org, cem@...nel.org, dchinner@...hat.com,
        hch@....de
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ojaswin@...ux.ibm.com,
        ritesh.list@...il.com, martin.petersen@...cle.com, tytso@....edu,
        linux-ext4@...r.kernel.org, John Garry <john.g.garry@...cle.com>
Subject: [PATCH v6 02/13] iomap: comment on atomic write checks in iomap_dio_bio_iter()

Help explain the code.

Also clarify the comment for bio size check.

Signed-off-by: John Garry <john.g.garry@...cle.com>
---
 fs/iomap/direct-io.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 8c1bec473586..9d72b99cb447 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -350,6 +350,11 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
 		bio_opf |= REQ_OP_WRITE;
 
 		if (iter->flags & IOMAP_ATOMIC_HW) {
+			/*
+			* Ensure that the mapping covers the full write length,
+			* otherwise we will submit multiple BIOs, which is
+			* disallowed.
+			*/
 			if (length != iter->len)
 				return -EINVAL;
 			bio_opf |= REQ_ATOMIC;
@@ -449,7 +454,7 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
 		n = bio->bi_iter.bi_size;
 		if (WARN_ON_ONCE((bio_opf & REQ_ATOMIC) && n != length)) {
 			/*
-			 * This bio should have covered the complete length,
+			 * An atomic write bio must cover the complete length,
 			 * which it doesn't, so error. We may need to zero out
 			 * the tail (complete FS block), similar to when
 			 * bio_iov_iter_get_pages() returns an error, above.
-- 
2.31.1