-------- Original Message --------
Subject: Re: [PATCH RESEND v4 6/8] vfs: Add get_vfsmount_sb() function 
to get vfsmount from a given sb.
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Qu Wenruo <redacted>
Date: 2015年01月30日 10:09

On Fri, Jan 30, 2015 at 09:11:26AM +0800, Qu Wenruo wrote:

quoted

For the mounted ro case, that's not a problem, since if one instance
is mounted ro,
other instances are also mounted ro, so that's not a problem.

Not really.

root@satch:~# cd /tmp/
root@satch:~# mkdir /tm/a
root@satch:~# mount --bind /tmp/ /tmp/a
root@satch:~# mount --bind -o remount,ro /tmp/a
root@satch:~# mkdir /tmp/b
root@satch:~# mkdir /tmp/a/c
mkdir: cannot create directory '/tmp/a/c': Read-only file system
root@satch:~# stat /tmp/b /tmp/a/b
   File: '/tmp/b'
   Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: 806h/2054d      Inode: 257537      Links: 2
Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2015-01-29 21:03:35.000000000 -0500
Modify: 2015-01-29 21:03:35.000000000 -0500
Change: 2015-01-29 21:03:35.000000000 -0500
  Birth: -
   File: '/tmp/a/b'
   Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: 806h/2054d      Inode: 257537      Links: 2
Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2015-01-29 21:03:35.000000000 -0500
Modify: 2015-01-29 21:03:35.000000000 -0500
Change: 2015-01-29 21:03:35.000000000 -0500
  Birth: -
root@satch:~#

IOW, /tmp and /tmp/a bear the same filesystem (one from /dev/sda6), the former
is mounted r/w, the latter - r/o.

Oh, bind mount.....

I was so stupid to forget bind.

quoted

quoted

Assuming that your superblock is guaranteed to stay alive and usable for
whatever work you are trying to do, what's wrong with sb_want_write()?

Did you mean change the function name and it's parameter to
sb_want_write(sb) and sb_drop_write(sb).
That looks much better.
But I'm a little worried about just using sb_start_write() and
s_readonly_remount/s_flags to do the protection,
will it be enough?

Protection against what?  If superblock is r/w, it will stay r/w until you
do sb_drop_write(); if it's r/o, sb_want_write() will fail.  There might be
any number of vfsmounts over superblock; attempt to get write access via
vfsmount will succeed only of both the vfsmount and superblock are r/w -
mnt_want_write() does sb_want_write() and fails if that has failed.

Nice.
I'll add sb_want_write() and sb_drop_write().

Tons of thanks for all your advice!
Qu