linux-ext4 - [e2fsprogs PATCH] resize2fs: use directio when reading superblock

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Message-ID: <20230605225221.GA5737@templeofstupid.com>
Date:   Mon, 5 Jun 2023 15:52:21 -0700
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     linux-ext4@...r.kernel.org
Cc:     Theodore Ts'o <tytso@....edu>
Subject: [e2fsprogs PATCH] resize2fs: use directio when reading superblock

Invocations of resize2fs intermittently report failure due to superblock
checksum mismatches in this author's environment.  This might happen a few
times a week.  The following script can make this happen within minutes.
(It assumes /dev/nvme1n1 is available and not in use by anything else).

   #!/usr/bin/bash
   set -euxo pipefail

   while true
   do
           parted /dev/nvme1n1 mklabel gpt mkpart primary 2048s 2099200s
           sleep .5
           mkfs.ext4 /dev/nvme1n1p1
           mount -t ext4 /dev/nvme1n1p1 /mnt
           stress-ng --temp-path /mnt -D 4 &
           STRESS_PID=$!
           sleep 1
           growpart /dev/nvme1n1 1
           resize2fs /dev/nvme1n1p1
           kill $STRESS_PID
           wait $STRESS_PID
           umount /mnt
           wipefs -a /dev/nvme1n1p1
           wipefs -a /dev/nvme1n1
   done

After trying a few possible solutions, adding an O_DIRECT read to the open
path in resize2fs eliminated the occurrences on test systems. ext2fs_open2
uses a negative count value when calling io_channel_read_blk to get the
superblock.  According to unix_read_block, negative offsets are to be read
direct.  However, when strace-ing a program without this fix, the
underlying device was opened without O_DIRECT.  Adding the flags in the
patch ensures the device is opend with O_DIRECT and that the superblock
read appears consistent.

Signed-off-by: Krister Johansen <kjlx@...pleofstupid.com>
---
 resize/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/resize/main.c b/resize/main.c
index 94f5ec6d..b98af384 100644
--- a/resize/main.c
+++ b/resize/main.c
@@ -410,7 +410,7 @@ int main (int argc, char ** argv)
 	if (!(mount_flags & EXT2_MF_MOUNTED) && !print_min_size)
 		io_flags = EXT2_FLAG_RW | EXT2_FLAG_EXCLUSIVE;

-	io_flags |= EXT2_FLAG_64BITS | EXT2_FLAG_THREADS;
+	io_flags |= EXT2_FLAG_64BITS | EXT2_FLAG_THREADS | EXT2_FLAG_DIRECT_IO;
 	if (undo_file) {
 		retval = resize2fs_setup_tdb(device_name, undo_file, &io_ptr);
 		if (retval)
-- 
2.25.1