linux-ext4 - [PATCH 03/16] fuse2fs: always use directio disk reads with fuse2fs

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Message-ID: <174787198487.1484996.12546102480862938973.stgit@frogsfrogsfrogs>
Date: Wed, 21 May 2025 17:11:37 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: John@...ves.net, linux-ext4@...r.kernel.org, miklos@...redi.hu,
 joannelkoong@...il.com, bernd@...ernd.com, linux-fsdevel@...r.kernel.org
Subject: [PATCH 03/16] fuse2fs: always use directio disk reads with fuse2fs

From: Darrick J. Wong <djwong@...nel.org>

In iomap mode, the kernel writes file data directly to the block device
and does not flush the bdev page cache.  We must open the filesystem in
directio mode to avoid cache coherency issues when reading file data
blocks.  If we can't open the bdev in directio mode, we must not use
iomap.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse2fs.c |   26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)


diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index 92a80753f4f1e8..91c0da096bef9c 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -988,8 +988,14 @@ static errcode_t confirm_iomap(struct fuse_conn_info *conn, struct fuse2fs *ff)
 
 	return 0;
 }
+
+static int iomap_enabled(const struct fuse2fs *ff)
+{
+	return ff->iomap_state == IOMAP_ENABLED;
+}
 #else
 # define confirm_iomap(...)	(0)
+# define iomap_enabled(...)	(0)
 #endif
 
 static void *op_init(struct fuse_conn_info *conn
@@ -1001,6 +1007,9 @@ static void *op_init(struct fuse_conn_info *conn
 	struct fuse_context *ctxt = fuse_get_context();
 	struct fuse2fs *ff = (struct fuse2fs *)ctxt->private_data;
 	ext2_filsys fs = ff->fs;
+#ifdef HAVE_FUSE_IOMAP
+	int was_directio = ff->directio;
+#endif
 	errcode_t err;
 	int ret;
 
@@ -1023,6 +1032,15 @@ static void *op_init(struct fuse_conn_info *conn
 	if (ff->iomap_state != IOMAP_DISABLED &&
 	    fuse_set_feature_flag(conn, FUSE_CAP_IOMAP))
 		ff->iomap_state = IOMAP_ENABLED;
+	/*
+	 * In iomap mode, the kernel writes file data directly to the block
+	 * device and does not flush the bdev page cache.  We must open the
+	 * filesystem in directio mode to avoid cache coherency issues when
+	 * reading file data.  If we can't open the bdev in directio mode, we
+	 * must not use iomap.
+	 */
+	if (iomap_enabled(ff))
+		ff->directio = 1;
 #endif
 
 #if FUSE_VERSION >= FUSE_MAKE_VERSION(3, 0)
@@ -1038,6 +1056,14 @@ static void *op_init(struct fuse_conn_info *conn
 	 */
 	if (!fs) {
 		err = open_fs(ff, 0);
+#ifdef HAVE_FUSE_IOMAP
+		if (err && iomap_enabled(ff) && !was_directio) {
+			fuse_unset_feature_flag(conn, FUSE_CAP_IOMAP);
+			ff->iomap_state = IOMAP_DISABLED;
+			ff->directio = 0;
+			err = open_fs(ff, 0);
+		}
+#endif
 		if (err)
 			goto mount_fail;
 		fs = ff->fs;