Re: filecache/swapcache questions

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Thu, 17 Jun 1999 17:20:10 -0700 (PDT), kanoj@google.engr.sgi.com
(Kanoj Sarcar) said:

> Interesting scenario ... unfortunately, I am getting confused.
> I am trying to lay out the steps in your example here:
 
> Step 1:  P1 and P2 sharing a page which is not in core, is out on
> swap at swap handle X, swap_count(X) = 2 (P1 + P2)

> Step 2: P1 writes to page.

>         Step 2a: swap_in reads in the page into core into page A, 
> page_count(A) = 2 (swapcache + P1), A.offset = X,  
> swap_count (X)= 2 (P2 + swapcache)

Yes.  Exactly.

> So, what am I missing, since your example does not end up with 
> page_count = 1 and swap_count(page offset/swaphandle) = 1?

> I did give an alternative scenario involving an exitting process,
> do you believe that one?

Yes --- I'd missed the fact that you wanted swap_count to be one as well
as page count.

> While I have your attention, I think I found a bug in the
> sys_swapoff algorithm ... basically, it needs to also look 
> at swap_lockmap. Say an exitting process fired off some async
> swap ins just before it exitted, and a bunch of these are in
> flight (swap_lockmaps are set, as are swap_map, from swapcache).
> The swap device gets deleted (with a printk warning message due
> to non zero swap_map count). Finally, the old async swap in's 
> start terminating, invoking swap_after_unlock_page. Interesting
> things could happen, depending on whether the swap id has been
> reallocated or not ... Is there any protection against this
> scenario?

Yes --- try_to_unuse calls read_swap_cache() with wait==1, so we always
wait for the IO to complete before swapoff can complete.  At least,
that's the theory. :)

--Stephen
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