Panic in gart_map_sg

[Amit Shah]

I get a kernel panic at gart_map_sg+0x201/0x470, which is around here:

	out++;
	flush_gart();
	if (out < nents) {
		sgmap = sg_next(sgmap);
		sgmap->dma_length = 0;
	}
	return out;

8e5:   e8 d6 fc ff ff          callq  5c0 <flush_gart>
 8ea:   ff c3                   inc    %ebx
 8ec:   41 39 df                cmp    %ebx,%r15d
 8ef:   0f 8e 9d 00 00 00       jle    992 <gart_map_sg+0x292>
 8f5:   31 c9                   xor    %ecx,%ecx
 8f7:   f6 45 00 02             testb  $0x2,0x0(%rbp)
 8fb:   0f 84 1f 02 00 00       je     b20 <gart_map_sg+0x420>
 901:   c7 41 18 00 00 00 00    movl   $0x0,0x18(%rcx)
 908:   e9 85 00 00 00          jmpq   992 <gart_map_sg+0x292>
 90d:   0f 1f 00                nopl   (%rax)
 910:   44 89 e0                mov    %r12d,%eax
 913:   29 f0                   sub    %esi,%eax
 915:   45 85 d2                test   %r10d,%r10d
 918:   89 c6                   mov    %eax,%esi
 91a:   0f 85 02 ff ff ff       jne    822 <gart_map_sg+0x122>
 920:   ff ce                   dec    %esi
 922:   75 65                   jne    989 <gart_map_sg+0x289>


My system is an AMD Opteron 1216. I'm not sure if only adding a check
for sgmap != NULL after sg_next is the right fix.

-- 
Amit Shah
http://www.amitshah.net/
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