[PATCH] Add +~800M crashkernel explaination

Tue Dec 13 19:08:01 PST 2016

On 12/10/2016 at 01:20 PM, Robert LeBlanc wrote:
> On Fri, Dec 9, 2016 at 7:49 PM, Baoquan He <bhe at redhat.com> wrote:
>> On 12/09/16 at 05:22pm, Robert LeBlanc wrote:
>>> When trying to configure crashkernel greater than about 800 MB, the
>>> kernel fails to allocate memory on x86 and x86_64. This is due to an
>>> undocumented limit that the crashkernel and other low memory items must
>>> be allocated below 896 MB unless the ",high" option is given. This
>>> updates the documentation to explain this and what I understand the
>>> limitations to be on the option.
>> This is true, but not very accurate. You found it's about 800M, it's
>> becasue usually the current kernel need about 40M space to run, and some
>> extra reservation before reserve_crashkernel invocation, another ~10M.
>> However it's normal case, people may build modules into or have some
>> special code to bloat kernel. This patch makes sense to address the
>> low|high issue, it might be not good so determined to say ~800M.
> My testing showed that I could go anywhere from about 830M to 880M,
> depending on distro, kernel version, and stuff that you mentioned. I
> just thought some rule of thumb of when to consider using high would
> be good. People may not think that 800 MB is 'large' when you have 512
> GB of RAM for instance. I thought about making 512 MB be the rule of
> thumb, but you can do a lot with ~300 MB.

Hi Robert,

I think you are correct.

For x86, the kernel uses memblock to locate the proper range starts from 16MB to some "end",
without "high" prefix, "end" is CRASH_ADDR_LOW_MAX, otherwise CRASH_ADDR_HIGH_MAX.

You can find the definition for both 32-bit and 64-bit:
#ifdef CONFIG_X86_32
# define CRASH_ADDR_LOW_MAX (512 << 20)
# define CRASH_ADDR_HIGH_MAX    (512 << 20)
#else
# define CRASH_ADDR_LOW_MAX (896UL << 20)
# define CRASH_ADDR_HIGH_MAX    MAXMEM
#endif

as some memory was already allocated by the kernel, which means it's highly likely to get a reservation
failure after specifying a crashkernel value near 800MB(for x86_64) which was what you met. But we can't
get the exact threshold, but it would be better if there is some explanation accordingly in the document.

>
> I'm happy to adjust the wording, what would you recommend? Also, I'm
> not 100% sure that I got the cases covered correctly. I was surprised
> that I could not get it to work with the "new" format with the
> multiple ranges, and that specifying an offset would't work either,
> although the offset kind of makes sense. Do you know for sure that it
> doesn't work with ranges?
>
> I tried,
>
> crashkernel=256M-1G:128M,high,1G-4G:256M,high,4G-:512M,high
>
> and
>
> crashkernel=256M-1G:128M,1G-4G:256M,4G-:512M,high
>
> and neither worked. It seems that a better separator would be ';'
> instead of ',' for ranges, then you could specify options better. Kind
> of hard to change now.

For "crashkernel=range1:size1[,range2:size2,...][@offset]"
I'm afraid it doesn't support "high" prefix in the current implementation, so there is no guarantee.
I guess we can drop a note to eliminate the confusion.

Regards,
Xunlei

>>> Signed-off-by: Robert LeBlanc <robert at leblancnet.us>
>>> ---
>>>  Documentation/kdump/kdump.txt | 22 +++++++++++++++++-----
>>>  1 file changed, 17 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
>>> index b0eb27b..aa3efa8 100644
>>> --- a/Documentation/kdump/kdump.txt
>>> +++ b/Documentation/kdump/kdump.txt
>>> @@ -256,7 +256,9 @@ While the "crashkernel=size[@offset]" syntax is sufficient for most
>>>  configurations, sometimes it's handy to have the reserved memory dependent
>>>  on the value of System RAM -- that's mostly for distributors that pre-setup
>>>  the kernel command line to avoid a unbootable system after some memory has
>>> -been removed from the machine.
>>> +been removed from the machine. If you need to allocate more than ~800M
>>> +for x86 or x86_64 then you must use the simple format as the format
>>> +',high' conflicts with the separators of ranges.
>>>
>>>  The syntax is:
>>>
>>> @@ -282,11 +284,21 @@ Boot into System Kernel
>>>  1) Update the boot loader (such as grub, yaboot, or lilo) configuration
>>>     files as necessary.
>>>
>>> -2) Boot the system kernel with the boot parameter "crashkernel=Y at X",
>>> +2) Boot the system kernel with the boot parameter "crashkernel=Y[@X | ,high]",
>>>     where Y specifies how much memory to reserve for the dump-capture kernel
>>> -   and X specifies the beginning of this reserved memory. For example,
>>> -   "crashkernel=64M at 16M" tells the system kernel to reserve 64 MB of memory
>>> -   starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
>>> +   and X specifies the beginning of this reserved memory or ',high' to load in
>>> +   high memory. For example, "crashkernel=64M at 16M" tells the system
>>> +   kernel to reserve 64 MB of memory starting at physical address
>>> +   0x01000000 (16MB) for the dump-capture kernel.
>>> +
>>> +   Specifying "crashkernel=1G,high" tells the system kernel to reserve 1 GB
>>> +   of memory using high memory for the dump-capture kernel, there may also
>>> +   be some low memory allocated as well. If you need more than ~800M for
>>> +   the crash kernel to operate (volumes on FC/iSCSI, large volumes, systemd
>>> +   added to the previous, etc), you need to specify ',high' since without
>>> +   it crashkerenel has to try and fit under 896M along with some other
>>> +   items and will fail to allocate memory. High memory may only be relevant
>>> +   on x86 and x86_64.
>>>
>>>     On x86 and x86_64, use "crashkernel=64M at 16M".
>>>
>>> --
>>> 2.10.2
>>>
>>>
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> ----------------
> Robert LeBlanc
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