linux-ext4 - [PATCH 16/19] fuse4fs: don't use inode number translation when possible

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Message-ID: <175573714019.21970.16728228028493607539.stgit@frogsfrogsfrogs>
Date: Wed, 20 Aug 2025 18:19:52 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: John@...ves.net, bernd@...ernd.com, linux-fsdevel@...r.kernel.org,
 linux-ext4@...r.kernel.org, miklos@...redi.hu, joannelkoong@...il.com,
 neal@...pa.dev
Subject: [PATCH 16/19] fuse4fs: don't use inode number translation when
 possible

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

Prior to the integration of iomap into fuse, the fuse client (aka the
kernel) required that the root directory have an inumber of
FUSE_ROOT_ID, which is 1.  However, the ext2 filesystem defines the root
inode number to be EXT2_ROOT_INO, which is 2.  This dissonance means
that we have to have translator functions, and that any access to
inumber 1 (the ext2 badblocks file) will instead redirect to the root
directory.

That's horrible.  Use the new mount option to set the root directory
nodeid to EXT2_ROOT_INO so that we don't need this translation.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse4fs.c |   29 +++++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)


diff --git a/misc/fuse4fs.c b/misc/fuse4fs.c
index 304bac191e7c4c..5127712e19e6f9 100644
--- a/misc/fuse4fs.c
+++ b/misc/fuse4fs.c
@@ -263,6 +263,7 @@ struct fuse4fs {
 	uint8_t unmount_in_destroy;
 	uint8_t noblkdev;
 	uint8_t iomap_passthrough_options;
+	uint8_t translate_inums;
 
 	enum fuse4fs_opstate opstate;
 	int logfd;
@@ -324,17 +325,19 @@ struct fuse4fs {
 #define FUSE4FS_CHECK_CONTEXT_ABORT(ff) \
 	__FUSE4FS_CHECK_CONTEXT((ff), abort(), abort())
 
-static inline void fuse4fs_ino_from_fuse(ext2_ino_t *inop, fuse_ino_t fino)
+static inline void fuse4fs_ino_from_fuse(const struct fuse4fs *ff,
+					 ext2_ino_t *inop, fuse_ino_t fino)
 {
-	if (fino == FUSE_ROOT_ID)
+	if (ff->translate_inums && fino == FUSE_ROOT_ID)
 		*inop = EXT2_ROOT_INO;
 	else
 		*inop = fino;
 }
 
-static inline void fuse4fs_ino_to_fuse(fuse_ino_t *finop, ext2_ino_t ino)
+static inline void fuse4fs_ino_to_fuse(const struct fuse4fs *ff,
+				       fuse_ino_t *finop, ext2_ino_t ino)
 {
-	if (ino == EXT2_ROOT_INO)
+	if (ff->translate_inums && ino == EXT2_ROOT_INO)
 		*finop = FUSE_ROOT_ID;
 	else
 		*finop = ino;
@@ -350,7 +353,7 @@ static inline void fuse4fs_ino_to_fuse(fuse_ino_t *finop, ext2_ino_t ino)
 			fuse_reply_err((req), EIO); \
 			return; \
 		} \
-		fuse4fs_ino_from_fuse(ext2_inop, fuse_ino); \
+		fuse4fs_ino_from_fuse(fuse4fs_get(req), ext2_inop, fuse_ino); \
 	} while (0)
 
 static int __translate_error(ext2_filsys fs, ext2_ino_t ino, errcode_t err,
@@ -1723,7 +1726,7 @@ static int fuse4fs_stat_inode(struct fuse4fs *ff, ext2_ino_t ino,
 			statbuf->st_rdev = inodep->i_block[1];
 	}
 
-	fuse4fs_ino_to_fuse(&entry->ino, ino);
+	fuse4fs_ino_to_fuse(ff, &entry->ino, ino);
 	entry->generation = inodep->i_generation;
 	entry->attr_timeout = FUSE4FS_ATTR_TIMEOUT;
 	entry->entry_timeout = FUSE4FS_ATTR_TIMEOUT;
@@ -7101,6 +7104,7 @@ int main(int argc, char *argv[])
 		.iomap_state = IOMAP_UNKNOWN,
 		.iomap_dev = FUSE_IOMAP_DEV_NULL,
 #endif
+		.translate_inums = 1,
 	};
 	errcode_t err;
 	FILE *orig_stderr = stderr;
@@ -7206,6 +7210,19 @@ int main(int argc, char *argv[])
 		goto out;
 	}
 
+	if (iomap_detected) {
+		/*
+		 * The root_nodeid mount option was added when iomap support
+		 * was added to fuse.  This enables us to control the root
+		 * nodeid in the kernel, which enables a 1:1 translation of
+		 * ext2 to kernel inumbers.
+		 */
+		snprintf(extra_args, BUFSIZ, "-oroot_nodeid=%d",
+			 EXT2_ROOT_INO);
+		fuse_opt_add_arg(&args, extra_args);
+		fctx.translate_inums = 0;
+	}
+
 	if (!fctx.cache_size)
 		fctx.cache_size = default_cache_size();
 	if (fctx.cache_size) {