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Date: Fri, 14 Jun 2024 11:14:27 +0800
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From: Li Lingfeng <lilingfeng@huaweicloud.com>
Subject: Re: [PATCH RFC 2/2] NFSv4: set sb_flags to second superblock
To: dhowells@redhat.com, marc.dionne@auristor.com, raven@themaw.net,
 gregkh@linuxfoundation.org, rafael@kernel.org, viro@zeniv.linux.org.uk,
 brauner@kernel.org, jack@suse.cz, miklos@szeredi.hu,
 trond.myklebust@hammerspace.com, anna@kernel.org, sfrench@samba.org,
 pc@manguebit.com, ronniesahlberg@gmail.com, sprasad@microsoft.com,
 tom@talpey.com, bharathsm@microsoft.com, djwong@kernel.org
Cc: linux-afs@lists.infradead.org, linux-kernel@vger.kernel.org,
 autofs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
 linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org,
 samba-technical@lists.samba.org, yi.zhang@huawei.com, yangerkun@huawei.com,
 zhangxiaoxu5@huawei.com, lilingfeng3@huawei.com
References: <20240604112636.236517-1-lilingfeng@huaweicloud.com>
 <20240604112636.236517-3-lilingfeng@huaweicloud.com>
In-Reply-To: <20240604112636.236517-3-lilingfeng@huaweicloud.com>
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I think this may be a problem, but I'm unable to come up with a suitable 
solution. Would you mind providing some suggestions?

在 2024/6/4 19:26, Li Lingfeng 写道:
> From: Li Lingfeng <lilingfeng3@huawei.com>
>
> During the process of mounting an NFSv4 client, two superblocks will be
> created in sequence. The first superblock corresponds to the root
> directory exported by the server, and the second superblock corresponds to
> the directory that will be actually mounted. The first superblock will
> eventually be destroyed.
> The flag passed from user mode will only be passed to the first
> superblock, resulting in the actual used superblock not carrying the flag
> passed from user mode(fs_context_for_submount() will set sb_flags as 0).
>
> If the 'ro' parameter is used in two consecutive mount commands, only the
> first execution will create a new vfsmount, and the kernel will return
> EBUSY on the second execution. However, if a remount command with the 'ro'
> parameter is executed between the two mount commands, both mount commands
> will create new vfsmounts.
>
> The superblock generated after the first mount command does not have the
> 'ro' flag, and the read-only status of the file system is implemented by
> checking the read-only flag of the vfsmount. After executing the remount
> command, the 'ro' flag will be added to the superblock. When the second
> mount command is executed, the comparison result between the superblock
> with the 'ro' flag and the fs_context without the flag in the
> nfs_compare_mount_options() function will be different, resulting in the
> creation of a new vfsmount.
>
> This problem can be reproduced by performing the following operations:
> mount -t nfs -o ro,vers=4.0 192.168.240.250:/sdb /mnt/sdb
> mount -t nfs -o remount,ro,vers=4.0 192.168.240.250:/sdb /mnt/sdb
> mount -t nfs -o ro,vers=4.0 192.168.240.250:/sdb /mnt/sdb
> Two vfsmounts are generated:
> [root@localhost ~]# mount | grep nfs
> 192.168.240.250:/sdb on /mnt/sdb type nfs4 (ro,relatime,vers=4.0,
> rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,
> sec=sys,clientaddr=192.168.240.251,local_lock=none,addr=192.168.240.250)
> 192.168.240.250:/sdb on /mnt/sdb type nfs4 (ro,relatime,vers=4.0,
> rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,
> sec=sys,clientaddr=192.168.240.251,local_lock=none,addr=192.168.240.250)
>
> Fix this by setting sb_flags to second superblock.
>
> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
> ---
>   fs/nfs/namespace.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
> index 887aeacedebd..8b3d75af60d4 100644
> --- a/fs/nfs/namespace.c
> +++ b/fs/nfs/namespace.c
> @@ -158,7 +158,7 @@ struct vfsmount *nfs_d_automount(struct path *path, unsigned int sb_flags)
>   	/* Open a new filesystem context, transferring parameters from the
>   	 * parent superblock, including the network namespace.
>   	 */
> -	fc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry, 0);
> +	fc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry, sb_flags);
>   	if (IS_ERR(fc))
>   		return ERR_CAST(fc);
>