Hi all,
Recently, swap got broken. When it has to be used, the kernel issue the
following line numerous times :
swap_free: Bad swap offset entry 00100009
It eventually panic with useless backtrace. Note that the offset entry
is always the same number.
This can be reproduced easily by booting with mem=64m and start
compiling something like the kernel.
I've been bisecting this and I found out it has been introduced in
4e60c86bd9e5a7110ed28874d0b6592186550ae8 and to be precise, the
following change in include/linux/mmdebug.h :
-#define VM_BUG_ON(cond) do { } while (0)
+#define VM_BUG_ON(cond) do { (void)(cond); } while (0)
While the above change looks harmless, it introduces a slight change in
mm/swap_state.c:154 in add_to_swap() : VM_BUG_ON(!PageUptodate(page));
If you comment out that line, the problem is gone.
PageUptodate() will call smp_rmb() being the memory barrier instruction.
Those tests were done with linux-2.6.36 and gcc-4.4.4.
Having the feeling that this might have been a gcc bug, I recompiled
the whole kernel with gcc-4.2.4 and the problem was gone.
Looking at the assembly output for add_to_swap(), there is a slight
difference.
- gcc-4.4.4 output :
add_to_swap:
.PROC
.CALLINFO FRAME=64,CALLS,SAVE_RP,ENTRY_GR=4
.ENTRY
stw %r2,-20(%r30)
stw,ma %r4,64(%r30)
stw %r3,-60(%r30)
ldw 0(%r26),%r28
copy %r26,%r3
ldw 0(%r26),%r28
bb,>=,n %r28,28,.L61
.L61:
b,l get_swap_page,%r2
...
- gcc-4.2.4 output :
add_to_swap:
.PROC
.CALLINFO FRAME=64,CALLS,SAVE_RP,ENTRY_GR=4
.ENTRY
stw %r2,-20(%r30)
stw,ma %r4,64(%r30)
stw %r3,-60(%r30)
ldw 0(%r26),%r19
copy %r26,%r3
ldw 0(%r26),%r28
extrw,s,>= %r28,28,1,%r0
.L76:
b,l get_swap_page,%r2
...
As you can see, gcc-4.2 uses extrw while gcc-4.4 uses bb.
What's the next step in order to troubleshoot this ?
Is this really a gcc bug or is the problem elsewhere ?
HTH,
Guy
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