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SubjectRe: Forced umount (was lazy umount)
Tigran Aivazian wrote:

> The forced umount patch has been available for ages and the latest version
> can be downloaded from:
>
> http://www.moses.uklinux.net/patches/forced-umount-2.4.9.patch
>
> If there are any issues with it please let me know.


Admittedly, I haven't tried it yet, but the one thing I can see that
looks like an issue in the context of my original request is that if a
filesystem's underlying device is having IO problems (bad hardware,
etc), a forced umount using this patch will potentially also lock up (in
a D state) trying to close up everything cleanly before unmounting it,
contributing to the problem instead of fixing it.

I certainly understand (and tend to agree with) Alexander Viro's opinion
that this is a 2.5 issue (my original post was just to make sure it was
pointed out that we do still need to work on this), mainly with the
following reasoning:

I see no reason why a properly functioning system should ever need to
truly force a umount. Under normal conditions, if one really needs to
do an emergency umount, it should be possible to use fuser/kill/etc to
clean up any processes using the filesystem from userland and then
perform a normal umount to cleanly unmount the filesystem in question
(or, with lazy umount, this could conceivably even be done in the
reverse order). The only reason for really-I-mean-it-forcing a umount
is if there is some problem which has caused one or more processes to
get permanently stuck in a state where they can't be killed (i.e. D
state), and thus can't release their hold on the filesystem. Ignoring
NFS for the moment, assuming that the block device drivers are written
correctly, there should be no way for anything to get stuck in disk-wait
for an extended period of time unless there is an actual physical
hardware problem preventing IO (I believe.. correct me if I'm wrong).

If there is a physical failure preventing IO to the underlying device,
then it is very likely that any attempts made by the umount call to read
from or write to the device will also block (unless there are some
special hooks into the block device drivers to avoid this, which I
assume there aren't). Therefore, if a forced umount is actually
required, it must not attempt to do any IO to the filesystem in question
either, and must instead just tear down the kernel's structures
associated with it, leaving the filesystem dirty on the disk, possibly
losing data in the process. This is why I had said in my first message
that this should really only be a very last resort.

Now, a version of this patch which didn't attempt to actually do any IO
on the device and modified umount (and presumably the various fs
drivers) so it doesn't do any flushing, fs structure cleanup, etc, might
be able to adequately do this, but given the degree of unchartedness in
this territory, I can certainly sympathize with not wanting to put it
into 2.4.

That's not to say that what the forced umount patch does isn't kinda
nifty and convenient, and I would like to see this sort of functionality
too, but it still doesn't really address the problem I was bringing up..

-alex

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