Re: [BUG] kernel BUG at mm/vmacache.c:85!

[Hugh Dickins <hughd@google.com>]

On Thu, 1 May 2014, Srivatsa S. Bhat wrote:
> 
> I tried to recall the *exact* steps that I had carried out when I first
> hit the bug. I realized that I had actually used kexec to boot the new
> kernel. I had originally booted into a 3.7.7 kernel that happens to be
> on that machine, and then kexec()'ed 3.15-rc3 on it. And that had caused
> the kernel crash. Fresh boots of 3.15-rc3, as well as kexec from 3.15+
> to itself, seems to be pretty robust and has never resulted in any bad
> behavior (this is why I couldn't reproduce the issue earlier, since I was
> doing fresh boots of 3.15-rc).
> 
> So I tried the same recipe again (boot into 3.7.7 and kexec into 3.15-rc3+)
> and I got totally random crashes so far, once in sys_kill and two times in
> exit_mmap. So I guess the bug is in 3.7.x and probably 3.15-rc is fine after
> all...

I don't know if we can conclude the bug is in 3.7 rather than 3.15.

I spent a little while yesterday looking at your register dumps,
and applying scripts/decodecode to your Code lines.  I did notice a
pattern to the general protection faulting addresses, and the dumps
you show today confirm that pattern (but with "1e000000" at the top
instead of yesterday's "9e000000").

Sorry, I really cannot spend more time on this, but thought I should
at least throw out my observation before moving on.  Here I've simply
grepped out the lines with the significant pattern (and at least one
of these lines is essentially a repetition of the line before, value
moved from one register to another with offset subtracted; oh, and
that R12 line, "it" has been added on to the vsize acct_collect()
already accumulated).

RAX: 9e00000005f9e8fd RBX: 000000000000000b RCX: 0000000000000001
RAX: 9e00000005f9e5fd RBX: ffff881031a0c2f8 RCX: ffff88203d52ba40
RDX: 000000000000001e RSI: 9e00000005f9e5a5 RDI: ffff881031a0c2f8
R10: 0000000000000000 R11: 00000000000027d5 R12: 9e000000069d62fd
BUG: Bad page map in process kdump  pte:1e00000005f98701 pmd:1031489067
BUG: Bad page map in process kdump  pte:1e00000005f98701 pmd:103420b067
R13: 0000000000000004 R14: 1e00000005f93403 R15: 000000000000000a

That this corruption likes to attack mm structures (vmas yesterday,
page tables today) does make me wonder whether mm is to blame.

Hugh

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