Re: Corrupted low memory in v3.9+

[Olof Johansson]

On Thu, Oct 17, 2013 at 1:39 PM, Olof Johansson <olof@xxxxxxxxx> wrote:
> 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.

*prod*

So, got a preference on solution for this? The warning seems harmless
but still annoying to get used to ignoring false positives, etc.

Disable the low memory checker by default? Hide it behind a debug
option (runtime or build time)?


-Olof
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