Re: Corrupted low memory in v3.9+

[Olof Johansson]

On Thu, Oct 17, 2013 at 12:39 PM, H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> wrote:
> On 10/17/2013 11:57 AM, Olof Johansson wrote:
>>
>> And the low memory checker never even ran before, since it had nothing
>> to check. Earlier the lower reserved region would be included in the
>> e820-reserved area if I read the code correctly, and now it's just
>> marked reserved by the memblock code.
>>
>> I guess it could be argued either way whether this is a regression or
>> not; but at the end of the day we now have systems where this warning
>> pops when it didn't use to. :(
>>
>
> I'm wondering if this is a problem with the low memory checker (the
> residual value of which I have to admit to being skeptical of) or
> something else.

There's a chance that it's a valid trip of the low-memory checker,
i.e. that we do have a bios (or more likely smm), that stomps on that
memory -- it was never checked for in the past and definitely not
warned about. I'm not sure if that was intentional behavior or not (to
not check this area), I lack history on the topic.

> Could you boot the box with "debug memblock=debug" and earlyprintk
> turned on and send the boot output?

Ah, yes, I did verify that the first 64K were indeed set aside as
reserved by doing just that:

[    0.000000] MEMBLOCK configuration:
[    0.000000]  memory size = 0x7c750000 reserved size = 0xb05000
[    0.000000]  memory.cnt  = 0x6
[    0.000000]  memory[0x0] [0x00000000010000-0x0000000009ffff], 0x90000 bytes
[    0.000000]  memory[0x1] [0x00000000100000-0x00000000efffff], 0xe00000 bytes
[    0.000000]  memory[0x2] [0x00000001000000-0x0000001fffffff],
0x1f000000 bytes
[    0.000000]  memory[0x3] [0x00000020200000-0x0000003fffffff],
0x1fe00000 bytes
[    0.000000]  memory[0x4] [0x00000040200000-0x0000007c6bffff],
0x3c4c0000 bytes
[    0.000000]  memory[0x5] [0x00000100000000-0x000001005fffff], 0x600000 bytes
[    0.000000]  reserved.cnt  = 0x2
[    0.000000]  reserved[0x0] [0x0000000009f000-0x000000000fffff], 0x61000 bytes
[    0.000000]  reserved[0x1] [0x00000001000000-0x00000001aa3fff],
0xaa4000 bytes
[    0.000000] memblock_reserve: [0x00000000099000-0x0000000009f000]
reserve_real_mode+0x61/0x87
[    0.000000] Base memory trampoline at [ffff880000099000] 99000 size 24576
[    0.000000] reserving inaccessible SNB gfx pages
[    0.000000] memblock_reserve: [0x00000000000000-0x00000000100000]
setup_arch+0xa2d/0xa41
[...]

Unfortunately x86 doesn't keep the memblock structures around, so
there's no way to verify after booting in debugfs, but based on the
above it should have been reserved properly.


-Olof
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