Re: [PATCH] Add +~800M crashkernel explaination

From: Xunlei Pang
Date: Tue Dec 13 2016 - 22:22:01 EST

On 12/14/2016 at 11:08 AM, Xunlei Pang wrote:
> On 12/10/2016 at 01:20 PM, Robert LeBlanc wrote:
>> On Fri, Dec 9, 2016 at 7:49 PM, Baoquan He <bhe@xxxxxxxxxx> 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)
> #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.

But there is another point:
If you specify the base using crashkernel=size[KMG][@offset[KMG]], for example
"crashkernel=1024M@0x10000000", there is no such limitation, and you may get
a successful reservation. I have no idea why the design is so different.


>> 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@xxxxxxxxxxxxx>
>>>> ---
>>>> 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@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@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@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@16M".
>>>> --
>>>> 2.10.2
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>> ----------------
>> Robert LeBlanc
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