Re: [RFC] [PATCH] VM & VFS changes

On  2 Jun, Alexander Leidinger wrote:
> Don Lewis <truckman_at_FreeBSD.org> wrote:
> 
>>> What am I overlooking?
>>
>> Create a large, but nearly empty file system, /a
>                                   ^
>                           memory/swap based

I think that only the swap-backed case is problematical.

>> Mount a file system backed by a physical disk on /a/b
>> Create the file /a/b/c and configure it to be used as swap
>> Write a large amount of data to the file /a/d, which will overflow RAM
>> and be paged out to /a/b/c
>>
>> It won't be possible to disable swapping to /a/b/c because there is not
>> sufficient RAM to page in the data stored there.  It won't be possible
>> to unmount /a/b because /a/b/c is busy.  It won't be possible to unmount
>> /a because it is busy because /a/b is mounted on it.
> 
> Ah! Yes! I tend to forget this is possible. In my mind a memory/swap based FS
> as a "leaf" in the directory tree... or more correctly, the subtree below the
> memory/swap based FS isn't allowed to contain a mointpoint of a non
> memory/swap based FS. Any other use which may require to break this rule has
> to use symlinks instead and isn't allowed to break the rule.

I'm pretty sure that sysinstall violates that by using a memory-backed
fs for the root partition.  The file systems being installed-to are
mounted below this memory-backed fs.


> This helps to avoid some pitfalls.
> 
>> If the dependencies are tracked so that this configuration (swapping to
>> anything that is directly or indirectly dependent on a swap-backed file
>> system) can be forbidden, then either the algorithm that I suggested, or
> 
> I'm not sure if we should enforce this policy... I like it, but I think such
> a restriction should be configurable via sysctl (enabled by default).

We should either enforce this or document that doing it might be
undoable later and could cause a deadlock on shutdown.  It's only swap
depending on swap that is dangerous.  A swap file that has a dependency
on a swap-backed fs is the only problem.  A swap file that depends on a
memory-backed fs should be ok, though a swap file that resides on a
memory-backed fs should probably be forbidden as well.