On Thu, 7 Jun 2001, Bulent Abali wrote:

I tested your patch against 2.4.5.  It works.  No more lockups.  Without
the
patch it took 14 minutes 51 seconds to complete swapoff (this is to recover
1.5GB of
swap space).  During this time the system was frozen.  No keyboard, no
screen, etc. Practically locked-up.

With the patch there are no more lockups. Swapoff kept running in the
background.
This is a winner.

But here is the caveat: swapoff keeps burning 100% of the cycles until it
completes.

Yup. Wait a while until the dead swap cache issue is sorted out. 

When that finally happens, the time spent in swapoff will probably be
"acceptable".

This is not going to be a big deal during shutdowns.  Only when you enter
swapoff from
the command line it is going to be a problem.

I looked at try_to_unuse in swapfile.c.  I believe that the algorithm is
broken.

Yes. 

For each and every swap entry it is walking the entire process list
(for_each_task(p)).  It is also grabbing a whole bunch of locks
for each swap entry.  It might be worthwhile processing swap entries in
batches instead of one entry at a time.

The real fix is to make the processing the other way around --- go looking
into the pte's and from there do the swapins. 

Don't have the time to do everything, though. :) 

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