linux-ext4 - [PATCH 17/22] fuse2fs: avoid fuseblk mode if fuse-iomap support is likely

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Message-ID: <175279461340.715479.12244748944041540648.stgit@frogsfrogsfrogs>
Date: Thu, 17 Jul 2025 16:43:48 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: joannelkoong@...il.com, miklos@...redi.hu, John@...ves.net,
 linux-fsdevel@...r.kernel.org, bernd@...ernd.com, linux-ext4@...r.kernel.org,
 neal@...pa.dev
Subject: [PATCH 17/22] fuse2fs: avoid fuseblk mode if fuse-iomap support is
 likely

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

Since fuse in iomap mode guarantees that op_destroy will be called
before umount returns, we don't need to use fuseblk mode to get that
guarantee.  Disable fuseblk mode, which saves us the trouble of closing
and reopening the device.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse2fs.c |   22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)


diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index 9a62971f8dbba7..82b59c1ac89774 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -982,6 +982,8 @@ static errcode_t fuse2fs_open(struct fuse2fs *ff, int libext2_flags)
 	if (ff->directio)
 		flags |= EXT2_FLAG_DIRECT_IO;
 
+	dbg_printf(ff, "opening with flags=0x%x\n", flags);
+
 	err = ext2fs_open2(ff->device, options, flags, 0, 0, unix_io_manager,
 			   &ff->fs);
 	if (err == EPERM) {
@@ -6333,10 +6335,24 @@ static unsigned long long default_cache_size(void)
 	return ret;
 }
 
+#ifdef HAVE_FUSE_IOMAP
+static inline bool fuse2fs_discover_iomap(const struct fuse2fs *ff)
+{
+	if (ff->iomap_want == FT_DISABLE)
+		return false;
+
+	return fuse_discover_iomap();
+}
+#else
+# define fuse2fs_discover_iomap(...)	(false)
+#endif
+
 static inline bool fuse2fs_want_fuseblk(const struct fuse2fs *ff)
 {
 	if (ff->noblkdev)
 		return false;
+	if (fuse2fs_discover_iomap(ff))
+		return false;
 
 	return fuse2fs_on_bdev(ff);
 }
@@ -6499,6 +6515,12 @@ int main(int argc, char *argv[])
 		 * device) so that unmount will wait until op_destroy
 		 * completes.  If this is not a block device, we cannot use
 		 * fuseblk mode and should leave the filesystem open.
+		 *
+		 * However, fuse+iomap guarantees that op_destroy is called
+		 * before the filesystem is unmounted, so we don't need fuseblk
+		 * mode.  This save us the trouble of reopening the filesystem
+		 * later, and means that fuse2fs itself owns the exclusive lock
+		 * on the block device.
 		 */
 		fuse2fs_unmount(&fctx);