Hi,

I experienced several kernel BUGs while running the linux kernel version 
2.4.19
on a single cpu s390 machine with 2GB RAM and 256MB of swap space. All of 
these
BUGs happened at page_alloc.c in the function __free_pages_ok. In that 
function
there is the check
if (page->mapping) BUG();
which is exactly what happened. A page had a mapping but __free_pages_ok() 
got
called anyway. Looking at the backtrace I was able to see that this 
specific
BUG() occurred when page_cache_release() was called from the function
try_to_swap_out().

Looks to me that this function itself has a bug: after the drop_pte label 
it is
checked if the current page has a mapping. If this is true there is a jump 
to
the drop_pte label, where without any further checking 
page_cache_release() gets
called which will result in the above described BUG() if page_count(page) 
== 1.

Here is the output of the kernel (I removed all inline statements in 
vmscan.c):

kernel BUG at page_alloc.c:91! 
illegal operation: 0001 
CPU:    0    Not tainted 
           80042730 00000001 013c578c 6ce26e00 
           00000020 575a0001 6ce26e00 00000000 
           013c578c 80042388 80042730 6c7e13c8 
           00000000 00000000 00000000 00000000 
           00000000 00000000 00000000 00000000 
           00000000 00000000 00000000 00000000 
Call Trace: [<000430d2>] [<00040eec>] [<00041088>] [<00041132>] 
            [<000411da>] [<000412cc>] [<000413b0>] [<00041646>] 
Warning (Oops_read): Code line not seen, dumping what data is available

Trace; 000430d2 <__free_pages+52/58>
Trace; 00040eec <try_to_swap_out+224/284>
Trace; 00041088 <swap_out_pmd+13c/178>
Trace; 00041132 <swap_out_pgd+6e/a0>
Trace; 000411da <swap_out_vma+76/bc>
Trace; 000412cc <swap_out_mm+ac/d0>
Trace; 000413b0 <swap_out+c0/150>
Trace; 00041646 <shrink_cache+206/5c8>

regards,
Heiko

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