linux-ext4 - [e2fsprogs PATCH v2] resize2fs: use directio when reading superblock

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Message-ID: <20230911183905.GA1960@templeofstupid.com>
Date:   Mon, 11 Sep 2023 11:39:05 -0700
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: [e2fsprogs PATCH v2] 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>
---
v2:
  - Only set DIRECT_IO flag when resizing a mounted filesystem. (Feedback from
    Theodore Ts'o)
---
 resize/main.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/resize/main.c b/resize/main.c
index 94f5ec6d..f914c050 100644
--- a/resize/main.c
+++ b/resize/main.c
@@ -409,6 +409,8 @@ int main (int argc, char ** argv)

 	if (!(mount_flags & EXT2_MF_MOUNTED) && !print_min_size)
 		io_flags = EXT2_FLAG_RW | EXT2_FLAG_EXCLUSIVE;
+	if (mount_flags & EXT2_MF_MOUNTED)
+		io_flags |= EXT2_FLAG_DIRECT_IO;

 	io_flags |= EXT2_FLAG_64BITS | EXT2_FLAG_THREADS;
 	if (undo_file) {
-- 
2.25.1