Bug or Feature? Multiply mounted device

Bug or Feature? Multiply mounted device

[Ron House]

Hi all, apologies for posting here, but I need to ask the guys who know 
for sure. In my fstab, I have two entries for my DOS HD so I can mount 
it with or without CRLF mangling, as follows (sorry for the line splitting):

/dev/hda5   /d      vfat 
rw,suid,dev,async,users,umask=033,uid=1500,gid=1001,conv=b 0 0
/dev/hda5   /dc     vfat 
rw,suid,dev,async,users,umask=033,uid=1500,gid=1001,noauto,conv=a 0 0

I just noticed with my new Linux setup (kernel 2.4.20-8smp - don't know 
what all that means) that it will let me mount both at once, so I can 
see the partition as either /d or /dc at the same time. I may be going 
mad, but I seem to recall with my previous kernel that I couldn't do this.

My question: Bug or feature? If I write all sorts of stuff to both 
logical devices, will it correctly sort it all out on the same physical 
device, or will I wreck the partition? All help much appreciated!

-- 
Ron House     house@usq.edu.au
               http://www.sci.usq.edu.au/staff/house

Re: Bug or Feature? Multiply mounted device

[viro]

On Fri, Jul 16, 2004 at 02:19:04PM +1000, Ron House wrote:
> Hi all, apologies for posting here, but I need to ask the guys who know 
> for sure. In my fstab, I have two entries for my DOS HD so I can mount 
> it with or without CRLF mangling, as follows (sorry for the line splitting):
> 
> /dev/hda5   /d      vfat 
> rw,suid,dev,async,users,umask=033,uid=1500,gid=1001,conv=b 0 0
> /dev/hda5   /dc     vfat 
> rw,suid,dev,async,users,umask=033,uid=1500,gid=1001,noauto,conv=a 0 0
> 
> I just noticed with my new Linux setup (kernel 2.4.20-8smp - don't know 
> what all that means) that it will let me mount both at once, so I can 
> see the partition as either /d or /dc at the same time. I may be going 
> mad, but I seem to recall with my previous kernel that I couldn't do this.
> 
> My question: Bug or feature? If I write all sorts of stuff to both 
> logical devices, will it correctly sort it all out on the same physical 
> device, or will I wreck the partition? All help much appreciated!

You will get both mountpoints as aliases to the tree.  Note that all
fs-specific flags will be ignored on subsequent mounts right now.
Eventually different flags will give you -EBUSY.

BTW, conv= is silently ignored by vfat, so your conv=a is a no-op - that
stuff simply doesn't belong in kernel and CRLF mangling had been removed
quite a while ago (try to make it play nice with mmap(2) and see how soon
you'll run away screaming).