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I'm reading The Design of the Unix Operating System by Maurice J Bach, and have two questions regarding file system mounting:

  1. Why does the file system mount fail when the reference count of the mount point inode is greater than 1 in the file table?

  2. Why does Unix disallow mounting a file system at multiple points? (What bad things could happen if this were allowed?)

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This is related to programming. I got these questions while reading the design of the unix operating sytem by maurice J bach. I dont know why people voted to close this topic! –  user40559 Jul 25 '10 at 6:31
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2 Answers

The HP (HP-UX) man page for mount(2) says:

If mount() fails, errno is set to one of the following values.

  • [EACCES] A component of the path prefix denies search permission.
  • [EBUSY] path is currently mounted on, is someone's current working directory, or is otherwise busy.
  • [EBUSY] The file system associated with fs is currently mounted.

You get the first EBUSY when your question (1) applies because:

  • if the directory is already a mount point, you lose access to the previously mounted directory, which makes the prior mount irrelevant.
  • if the directory (say /some/where) is some process's current directory, you have a process with a different view of the contents of /some/where; newcomers see what's on the mounted file system, but the old processes see what was in the mounted-upon directory.

You get the second EBUSY to answer your question (2) when the file system is already mounted - in other words, you can't mount it twice. This is a good thing - there would be a dreadful danger of confusion if two separate mount points both went around assuming they had exclusive access to the superblock etc when it was in fact shared. It would also be confusing if creating a file /some/where/newfile also simultaneously created /opt/other/newfile because the same device was mounted on both /some/where and /opt/other.

I haven't checked the AIX, Solaris, Linux, MacOS X, BSD man pages for mount(2), but I expect the behaviour to be the same.

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@Jonathan thank you very much –  user40559 Jul 25 '10 at 19:05
    
@Neil Smithline: Please justify your 'many' variants by citing at least one non-Linux variant that provides what you describe: '25 July 2012 Update: Many Unix/Linux variants now support the -o union option that allows two filesystems to share a mount point. The second mount shadows the first in that all file accesses take place first in the new mount and then in the original mount. Also, file creation happens in the new mount.' Wikipedia has a brief outline but little specificity about which o/s other than Plan 9 support them. –  Jonathan Leffler Jul 26 '12 at 4:08
    
FreeBSD supports that option: freebsd.org/cgi/… –  jlliagre Jul 26 '12 at 6:16
    
Two of about seven — an exaggerated version of many. I prefer that the addendum is a comment (or a separate answer) than a modification of my answer. –  Jonathan Leffler Jul 26 '12 at 6:34
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This isn't a direct answer, but you can get behavior similar to mounting in two places by using mount --bind

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