linux-ext4 - [PATCH 22/22] fuse2fs: configure block device block size

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Message-ID: <175279461430.715479.13971213588209519937.stgit@frogsfrogsfrogs>
Date: Thu, 17 Jul 2025 16:45:07 -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 22/22] fuse2fs: configure block device block size

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

Set the blocksize of the block device to the filesystem blocksize.
This prevents the bdev pagecache from caching file data blocks that
iomap will read and write directly.  Cache duplication is dangerous.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse2fs.c |   40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)


diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index b5f665ada36991..d0478af036a25e 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -5683,6 +5683,42 @@ static off_t fuse2fs_max_size(struct fuse2fs *ff, off_t upper_limit)
 	return res;
 }
 
+/*
+ * Set the block device's blocksize to the fs blocksize.
+ *
+ * This is required to avoid creating uptodate bdev pagecache that aliases file
+ * data blocks because iomap reads and writes directly to file data blocks.
+ */
+static int fuse2fs_set_bdev_blocksize(struct fuse2fs *ff, int fd)
+{
+	int blocksize = ff->fs->blocksize;
+	int set_error;
+	int ret;
+
+	ret = ioctl(fd, BLKBSZSET, &blocksize);
+	if (!ret)
+		return 0;
+
+	/*
+	 * Save the original errno so we can report that if the block device
+	 * blocksize isn't set in an agreeable way.
+	 */
+	set_error = errno;
+
+	ret = ioctl(fd, BLKBSZGET, &blocksize);
+	if (ret)
+		goto out_bad;
+
+	if (blocksize > ff->fs->blocksize)
+		set_error = -EINVAL;
+
+	return 0;
+out_bad:
+	err_printf(ff, "%s: cannot set blocksize %u: %s\n", __func__,
+		   blocksize, strerror(set_error));
+	return -EIO;
+}
+
 static errcode_t fuse2fs_iomap_config_devices(struct fuse_context *ctxt,
 					      struct fuse2fs *ff)
 {
@@ -5695,6 +5731,10 @@ static errcode_t fuse2fs_iomap_config_devices(struct fuse_context *ctxt,
 	if (err)
 		return err;
 
+	ret = fuse2fs_set_bdev_blocksize(ff, fd);
+	if (ret)
+		return ret;
+
 	ret = fuse_iomap_add_device(se, fd, 0);
 
 	dbg_printf(ff, "%s: registering iomap dev fd=%d ret=%d iomap_dev=%u\n",