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Date:   Sun, 30 Aug 2020 21:45:11 -0400
From:   Qian Cai <>
        Qian Cai <>
Subject: [PATCH v2] iomap: Fix WARN_ON_ONCE() from unprivileged users

It is trivial to trigger a WARN_ON_ONCE(1) in iomap_dio_actor() by
unprivileged users which would taint the kernel, or worse - panic if
panic_on_warn or panic_on_taint is set. Hence, just convert it to
pr_warn_ratelimited() to let users know their workloads are racing.
Thanks Dave Chinner for the initial analysis of the racing reproducers.

Signed-off-by: Qian Cai <>

v2: Record the path, pid and command as well.

 fs/iomap/direct-io.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c1aafb2ab990..66a4502ef675 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -374,6 +374,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 		void *data, struct iomap *iomap, struct iomap *srcmap)
 	struct iomap_dio *dio = data;
+	char pathname[128], *path;
 	switch (iomap->type) {
 	case IOMAP_HOLE:
@@ -389,7 +390,21 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 		return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
+		/*
+		 * DIO is not serialised against mmap() access at all, and so
+		 * if the page_mkwrite occurs between the writeback and the
+		 * iomap_apply() call in the DIO path, then it will see the
+		 * DELALLOC block that the page-mkwrite allocated.
+		 */
+		path = file_path(dio->iocb->ki_filp, pathname,
+				 sizeof(pathname));
+		if (IS_ERR(path))
+			path = "(unknown)";
+		pr_warn_ratelimited("page_mkwrite() is racing with DIO read (iomap->type = %u).\n"
+				    "File: %s PID: %d Comm: %.20s\n",
+				    iomap->type, path, current->pid,
+				    current->comm);
 		return -EIO;

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