On 2025/2/21 16:26, Li Lingfeng wrote:
> In some scenarios, when mounting NFS, more than one superblock may be
> created. The final superblock used is the last one created, but only the
> first superblock carries the ro flag passed from user space. If a ro flag
> is added to the superblock via remount, it will trigger the issue
> described in Link[1].
>
> Link[2] attempted to address this by marking the superblock as ro during
> the initial mount. However, this introduced a new problem in scenarios
> where multiple mount points share the same superblock:
> [root@a ~]# mount /dev/sdb /mnt/sdb
> [root@a ~]# echo "/mnt/sdb *(rw,no_root_squash)" > /etc/exports
> [root@a ~]# echo "/mnt/sdb/test_dir2 *(ro,no_root_squash)" >> /etc/exports
> [root@a ~]# systemctl restart nfs-server
> [root@a ~]# mount -t nfs -o rw 127.0.0.1:/mnt/sdb/test_dir1 /mnt/test_mp1
> [root@a ~]# mount | grep nfs4
> 127.0.0.1:/mnt/sdb/test_dir1 on /mnt/test_mp1 type nfs4 (rw,relatime,...
> [root@a ~]# mount -t nfs -o ro 127.0.0.1:/mnt/sdb/test_dir2 /mnt/test_mp2
> [root@a ~]# mount | grep nfs4
> 127.0.0.1:/mnt/sdb/test_dir1 on /mnt/test_mp1 type nfs4 (ro,relatime,...
> 127.0.0.1:/mnt/sdb/test_dir2 on /mnt/test_mp2 type nfs4 (ro,relatime,...
> [root@a ~]#
>
> When mounting the second NFS, the shared superblock is marked as ro,
> causing the previous NFS mount to become read-only.
>
> To resolve both issues, the ro flag is no longer applied to the superblock
> during remount. Instead, the ro flag on the mount is used to control
> whether the mount point is read-only.
>
> Fixes: 281cad46b34d ("NFS: Create a submount rpc_op")
> Link[1]: https://lore.kernel.org/all/20240604112636.236517-3-lilingfeng@xxxxxxxxxxxxxxx/
> Link[2]: https://lore.kernel.org/all/20241130035818.1459775-1-lilingfeng3@xxxxxxxxxx/
> Signed-off-by: Li Lingfeng <lilingfeng3@xxxxxxxxxx>
> ---
> fs/nfs/super.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/fs/nfs/super.c b/fs/nfs/super.c
> index aeb715b4a690..f08e1d7fb179 100644
> --- a/fs/nfs/super.c
> +++ b/fs/nfs/super.c
> @@ -1047,6 +1047,7 @@ int nfs_reconfigure(struct fs_context *fc)
>
> sync_filesystem(sb);
>
> + fc->sb_flags &= ~SB_RDONLY;
What about change sb_flags_mask instead? Something like below,
fc->sb_flags_mask &= ~SB_RDONLY;
and I'd also suggested to add a comment to explain the reason in detail.
Thanks,
Yi.
