-------- 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 <v...@zeniv.linux.org.uk>
To: Qu Wenruo <quwen...@cn.fujitsu.com>
Date: 2015年01月30日 10:09
On Fri, Jan 30, 2015 at 09:11:26AM +0800, Qu Wenruo wrote:
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.
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
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