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

On Thu, 2 Jun 2005, 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
>
>> 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.
>
> 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).
>
>> your iterative algorithm should work. Your algorithm could even be
>> simplified by pulling the swap-off out of the loop.
>> 
>> unconfigure md's that are not busy
>> do {
>> 	foreach filesystem in reverse(mountlist) {
>> 		unmount filesystem
>> 		if (success and backed by md)
>> 			unconfigure md /* either swap or vnode backed */
>> 	}
>> } while (progress)
>> swap-off
>> do {
>> 	foreach filesystem in reverse(mountlist) {
>> 		unmount filesystem
>> 		if (success and backed by md)
>> 			unconfigure md /* only vnode backed remain */
>> 	}
>> } while (progress)
>> 
>> Removing the swap-off from the loop may avoid a lot of paging activity
>> because all the paged-out data from swap backed md's will be discarded
>> before swap is disabled.
>
> Sounds even better.

I will try to produce patches tonight for this. Stay tuned!

Cheers!
Andy

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