linux-ext4 - [PATCH 4/4] fuse4fs: set iomap backing device blocksize

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Message-ID: <175798163172.392148.13591414637945028939.stgit@frogsfrogsfrogs>
Date: Mon, 15 Sep 2025 18:09:26 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: miklos@...redi.hu, neal@...pa.dev, linux-fsdevel@...r.kernel.org,
 linux-ext4@...r.kernel.org, John@...ves.net, bernd@...ernd.com,
 joannelkoong@...il.com
Subject: [PATCH 4/4] fuse4fs: set iomap backing device blocksize

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

If we're running as an unprivileged iomap fuse server, we must ask the
kernel to set the blocksize of the block device.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 fuse4fs/fuse4fs.c |   41 +++++++++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 10 deletions(-)


diff --git a/fuse4fs/fuse4fs.c b/fuse4fs/fuse4fs.c
index 0e43e99c3c080d..40171a8cab5279 100644
--- a/fuse4fs/fuse4fs.c
+++ b/fuse4fs/fuse4fs.c
@@ -1371,6 +1371,21 @@ static int fuse4fs_service(struct fuse4fs *ff, struct fuse_session *se,
 
 	return 0;
 }
+
+int fuse4fs_service_set_bdev_blocksize(struct fuse4fs *ff, int dev_index)
+{
+	int ret;
+
+	ret = fuse_lowlevel_iomap_set_blocksize(ff->fusedev_fd, dev_index,
+						ff->fs->blocksize);
+	if (ret) {
+		err_printf(ff, "%s: cannot set blocksize %u: %s\n", __func__,
+			   ff->fs->blocksize, strerror(errno));
+		return -EIO;
+	}
+
+	return 0;
+}
 #else
 # define fuse4fs_is_service(...)		(false)
 # define fuse4fs_service_connect(...)		(0)
@@ -1382,6 +1397,7 @@ static int fuse4fs_service(struct fuse4fs *ff, struct fuse_session *se,
 # define fuse4fs_service_openfs(...)		(EOPNOTSUPP)
 # define fuse4fs_service_configure_iomap(...)	(EOPNOTSUPP)
 # define fuse4fs_service(...)			(EOPNOTSUPP)
+# define fuse4fs_service_set_bdev_blocksize(...) (EOPNOTSUPP)
 #endif
 
 static errcode_t fuse4fs_acquire_lockfile(struct fuse4fs *ff)
@@ -6798,21 +6814,19 @@ static int fuse4fs_iomap_config_devices(struct fuse4fs *ff)
 {
 	errcode_t err;
 	int fd;
+	int dev_index;
 	int ret;
 
 	err = io_channel_get_fd(ff->fs->io, &fd);
 	if (err)
 		return translate_error(ff->fs, 0, err);
 
-	ret = fuse4fs_set_bdev_blocksize(ff, fd);
-	if (ret)
-		return ret;
-
-	ret = fuse_lowlevel_iomap_device_add(ff->fuse, fd, 0);
-	if (ret < 0) {
-		dbg_printf(ff, "%s: cannot register iomap dev fd=%d, err=%d\n",
-			   __func__, fd, -ret);
-		return translate_error(ff->fs, 0, -ret);
+	dev_index = fuse_lowlevel_iomap_device_add(ff->fuse, fd, 0);
+	if (dev_index < 0) {
+		ret = -dev_index;
+		dbg_printf(ff, "%s: cannot register iomap dev fd=%d: %s\n",
+			   __func__, fd, strerror(ret));
+		return translate_error(ff->fs, 0, ret);
 	}
 
 	dbg_printf(ff, "%s: registered iomap dev fd=%d iomap_dev=%u\n",
@@ -6820,7 +6834,14 @@ static int fuse4fs_iomap_config_devices(struct fuse4fs *ff)
 
 	fuse4fs_configure_atomic_write(ff, fd);
 
-	ff->iomap_dev = ret;
+	if (fuse4fs_is_service(ff))
+		ret = fuse4fs_service_set_bdev_blocksize(ff, dev_index);
+	else
+		ret = fuse4fs_set_bdev_blocksize(ff, fd);
+	if (ret)
+		return ret;
+
+	ff->iomap_dev = dev_index;
 	return 0;
 }