Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Michael Holzheu]

Am Thu, 23 Mar 2017 17:23:53 +0800
schrieb Xunlei Pang :

> On 03/23/2017 at 04:48 AM, Michael Holzheu wrote:
> > Am Wed, 22 Mar 2017 12:30:04 +0800
> > schrieb Dave Young :
> >
> >> On 03/21/17 at 10:18pm, Eric W. Biederman wrote:
> >>> Dave Young  writes:
> >>>
> > [snip]
> >
>  I think makedumpfile is using it, but I also vote to remove the
>  CRASHTIME. It is better not to do this while crashing and a makedumpfile
>  userspace patch is needed to drop the use of it.
> 
> > As we are looking at reliability concerns removing CRASHTIME should make
> > everything in vmcoreinfo a boot time constant.  Which should simplify
> > everything considerably.
>  It is a nice improvement..
> >>> We also need to take a close look at what s390 is doing with vmcoreinfo.
> >>> As apparently it is reading it in a different kind of crashdump process.
> >> Yes, need careful review from s390 and maybe ppc64 especially about
> >> patch 2/3, better to have comments from IBM about s390 dump tool and ppc
> >> fadump. Added more cc.
> > On s390 we have at least an issue with patch 1/3. For stand-alone dump
> > and also because we create the ELF header for kdump in the new
> > kernel we save the pointer to the vmcoreinfo note in the old kernel on a
> > defined memory address in our absolute zero lowcore.
> >
> > This is done in arch/s390/kernel/setup.c:
> >
> > static void __init setup_vmcoreinfo(void)
> > {
> > mem_assign_absolute(S390_lowcore.vmcore_info, 
> > paddr_vmcoreinfo_note());
> > }
> >
> > Since with patch 1/3 paddr_vmcoreinfo_note() returns NULL at this point in
> > time we have a problem here.
> >
> > To solve this - I think - we could move the initialization to
> > arch/s390/kernel/machine_kexec.c:
> >
> > void arch_crash_save_vmcoreinfo(void)
> > {
> > VMCOREINFO_SYMBOL(lowcore_ptr);
> > VMCOREINFO_SYMBOL(high_memory);
> > VMCOREINFO_LENGTH(lowcore_ptr, NR_CPUS);
> > mem_assign_absolute(S390_lowcore.vmcore_info, 
> > paddr_vmcoreinfo_note());
> > }
> >
> > Probably related to this is my observation that patch 3/3 leads to
> > an empty VMCOREINFO note for kdump on s390. The note is there ...
> >
> > # readelf -n /var/crash/127.0.0.1-2017-03-22-21:14:39/vmcore | grep VMCORE
> >   VMCOREINFO   0x068e   Unknown note type: (0x)
> >
> > But it contains only zeros.
> 
> Yes, this is a good catch, I will do more tests.

Hello Xunlei,

After spending some time on this, I now understood the problem:

In patch 3/3 you copy vmcoreinfo into the control page before
machine_kexec_prepare() is called. For s390 we give back all the
crashkernel memory to the hypervisor before the new crashkernel
is loaded:

/*
 * Give back memory to hypervisor before new kdump is loaded
 */
static int machine_kexec_prepare_kdump(void)
{
#ifdef CONFIG_CRASH_DUMP
if (MACHINE_IS_VM)
diag10_range(PFN_DOWN(crashk_res.start),
 PFN_DOWN(crashk_res.end - crashk_res.start + 1));
return 0;
#else
return -EINVAL;
#endif
}

So after machine_kexec_prepare_kdump() the contents of your control page
is gone and therefore the vmcorinfo ELF note contains only zeros.

If you call kimage_crash_copy_vmcoreinfo() after
machine_kexec_prepare_kdump() the problem should be solved for s390.

Regards
Michael


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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Xunlei Pang]

On 03/23/2017 at 04:48 AM, Michael Holzheu wrote:
> Am Wed, 22 Mar 2017 12:30:04 +0800
> schrieb Dave Young :
>
>> On 03/21/17 at 10:18pm, Eric W. Biederman wrote:
>>> Dave Young  writes:
>>>
> [snip]
>
 I think makedumpfile is using it, but I also vote to remove the
 CRASHTIME. It is better not to do this while crashing and a makedumpfile
 userspace patch is needed to drop the use of it.

> As we are looking at reliability concerns removing CRASHTIME should make
> everything in vmcoreinfo a boot time constant.  Which should simplify
> everything considerably.
 It is a nice improvement..
>>> We also need to take a close look at what s390 is doing with vmcoreinfo.
>>> As apparently it is reading it in a different kind of crashdump process.
>> Yes, need careful review from s390 and maybe ppc64 especially about
>> patch 2/3, better to have comments from IBM about s390 dump tool and ppc
>> fadump. Added more cc.
> On s390 we have at least an issue with patch 1/3. For stand-alone dump
> and also because we create the ELF header for kdump in the new
> kernel we save the pointer to the vmcoreinfo note in the old kernel on a
> defined memory address in our absolute zero lowcore.
>
> This is done in arch/s390/kernel/setup.c:
>
> static void __init setup_vmcoreinfo(void)
> {
> mem_assign_absolute(S390_lowcore.vmcore_info, 
> paddr_vmcoreinfo_note());
> }
>
> Since with patch 1/3 paddr_vmcoreinfo_note() returns NULL at this point in
> time we have a problem here.
>
> To solve this - I think - we could move the initialization to
> arch/s390/kernel/machine_kexec.c:
>
> void arch_crash_save_vmcoreinfo(void)
> {
> VMCOREINFO_SYMBOL(lowcore_ptr);
> VMCOREINFO_SYMBOL(high_memory);
> VMCOREINFO_LENGTH(lowcore_ptr, NR_CPUS);
> mem_assign_absolute(S390_lowcore.vmcore_info, 
> paddr_vmcoreinfo_note());
> }
>
> Probably related to this is my observation that patch 3/3 leads to
> an empty VMCOREINFO note for kdump on s390. The note is there ...
>
> # readelf -n /var/crash/127.0.0.1-2017-03-22-21:14:39/vmcore | grep VMCORE
>   VMCOREINFO   0x068e   Unknown note type: (0x)
>
> But it contains only zeros.

Yes, this is a good catch, I will do more tests.

Thanks,
Xunlei

>
> Unfortunately I have not yet understood the reason for this.
>
> Michael
>
>
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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Michael Holzheu]

Am Wed, 22 Mar 2017 12:30:04 +0800
schrieb Dave Young :

> On 03/21/17 at 10:18pm, Eric W. Biederman wrote:
> > Dave Young  writes:
> > 

[snip]

> > > I think makedumpfile is using it, but I also vote to remove the
> > > CRASHTIME. It is better not to do this while crashing and a makedumpfile
> > > userspace patch is needed to drop the use of it.
> > >
> > >> 
> > >> As we are looking at reliability concerns removing CRASHTIME should make
> > >> everything in vmcoreinfo a boot time constant.  Which should simplify
> > >> everything considerably.
> > >
> > > It is a nice improvement..
> > 
> > We also need to take a close look at what s390 is doing with vmcoreinfo.
> > As apparently it is reading it in a different kind of crashdump process.
> 
> Yes, need careful review from s390 and maybe ppc64 especially about
> patch 2/3, better to have comments from IBM about s390 dump tool and ppc
> fadump. Added more cc.

On s390 we have at least an issue with patch 1/3. For stand-alone dump
and also because we create the ELF header for kdump in the new
kernel we save the pointer to the vmcoreinfo note in the old kernel on a
defined memory address in our absolute zero lowcore.

This is done in arch/s390/kernel/setup.c:

static void __init setup_vmcoreinfo(void)
{
mem_assign_absolute(S390_lowcore.vmcore_info, paddr_vmcoreinfo_note());
}

Since with patch 1/3 paddr_vmcoreinfo_note() returns NULL at this point in
time we have a problem here.

To solve this - I think - we could move the initialization to
arch/s390/kernel/machine_kexec.c:

void arch_crash_save_vmcoreinfo(void)
{
VMCOREINFO_SYMBOL(lowcore_ptr);
VMCOREINFO_SYMBOL(high_memory);
VMCOREINFO_LENGTH(lowcore_ptr, NR_CPUS);
mem_assign_absolute(S390_lowcore.vmcore_info, paddr_vmcoreinfo_note());
}

Probably related to this is my observation that patch 3/3 leads to
an empty VMCOREINFO note for kdump on s390. The note is there ...

# readelf -n /var/crash/127.0.0.1-2017-03-22-21:14:39/vmcore | grep VMCORE
  VMCOREINFO   0x068e   Unknown note type: (0x)

But it contains only zeros.

Unfortunately I have not yet understood the reason for this.

Michael


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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Hari Bathini]

Hi Dave,


On Wednesday 22 March 2017 10:00 AM, Dave Young wrote:

On 03/21/17 at 10:18pm, Eric W. Biederman wrote:

Dave Young  writes:


On 03/20/17 at 10:33pm, Eric W. Biederman wrote:

Xunlei Pang  writes:


As Eric said,
"what we need to do is move the variable vmcoreinfo_note out
of the kernel's .bss section.  And modify the code to regenerate
and keep this information in something like the control page.

Definitely something like this needs a page all to itself, and ideally
far away from any other kernel data structures.  I clearly was not
watching closely the data someone decided to keep this silly thing
in the kernel's .bss section."

This patch allocates extra pages for these vmcoreinfo_XXX variables,
one advantage is that it enhances some safety of vmcoreinfo, because
vmcoreinfo now is kept far away from other kernel data structures.

Can you preceed this patch with a patch that removes CRASHTIME from
vmcoreinfo?  If someone actually cares we can add a separate note that holds
a 64bit crashtime in the per cpu notes.

I think makedumpfile is using it, but I also vote to remove the
CRASHTIME. It is better not to do this while crashing and a makedumpfile
userspace patch is needed to drop the use of it.


As we are looking at reliability concerns removing CRASHTIME should make
everything in vmcoreinfo a boot time constant.  Which should simplify
everything considerably.

It is a nice improvement..

We also need to take a close look at what s390 is doing with vmcoreinfo.
As apparently it is reading it in a different kind of crashdump process.

Yes, need careful review from s390 and maybe ppc64 especially about
patch 2/3, better to have comments from IBM about s390 dump tool and ppc
fadump. Added more cc.


w.r.t powerpc/fadump, this patch-set works fine..

Thanks
Hari


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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Xunlei Pang]

On 03/22/2017 at 12:30 PM, Dave Young wrote:
> On 03/21/17 at 10:18pm, Eric W. Biederman wrote:
>> Dave Young  writes:
>>
>>> On 03/20/17 at 10:33pm, Eric W. Biederman wrote:
 Xunlei Pang  writes:

> As Eric said,
> "what we need to do is move the variable vmcoreinfo_note out
> of the kernel's .bss section.  And modify the code to regenerate
> and keep this information in something like the control page.
>
> Definitely something like this needs a page all to itself, and ideally
> far away from any other kernel data structures.  I clearly was not
> watching closely the data someone decided to keep this silly thing
> in the kernel's .bss section."
>
> This patch allocates extra pages for these vmcoreinfo_XXX variables,
> one advantage is that it enhances some safety of vmcoreinfo, because
> vmcoreinfo now is kept far away from other kernel data structures.
 Can you preceed this patch with a patch that removes CRASHTIME from
 vmcoreinfo?  If someone actually cares we can add a separate note that 
 holds
 a 64bit crashtime in the per cpu notes.  
>>> I think makedumpfile is using it, but I also vote to remove the
>>> CRASHTIME. It is better not to do this while crashing and a makedumpfile
>>> userspace patch is needed to drop the use of it.
>>>

By moving the CRASHTIME info to the cpu note of crashed cpu may be a good
way. In kdump kernel, notes of vmcore elfhdr will be merged into one big note
section, I don't know how makedumpfile or crash handle the big note section?
If they process the note in some order, breakage will definitely happen...

There is also a fadump may be affected.

Regards,
Xunlei

 As we are looking at reliability concerns removing CRASHTIME should make
 everything in vmcoreinfo a boot time constant.  Which should simplify
 everything considerably.
>>> It is a nice improvement..
>> We also need to take a close look at what s390 is doing with vmcoreinfo.
>> As apparently it is reading it in a different kind of crashdump process.
> Yes, need careful review from s390 and maybe ppc64 especially about
> patch 2/3, better to have comments from IBM about s390 dump tool and ppc
> fadump. Added more cc.
>
> Thanks
> Dave
>
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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Xunlei Pang]

On 03/22/2017 at 04:55 PM, Xunlei Pang wrote:
> On 03/21/2017 at 11:33 AM, Eric W. Biederman wrote:
>> Xunlei Pang  writes:
>>
>>> As Eric said,
>>> "what we need to do is move the variable vmcoreinfo_note out
>>> of the kernel's .bss section.  And modify the code to regenerate
>>> and keep this information in something like the control page.
>>>
>>> Definitely something like this needs a page all to itself, and ideally
>>> far away from any other kernel data structures.  I clearly was not
>>> watching closely the data someone decided to keep this silly thing
>>> in the kernel's .bss section."
>>>
>>> This patch allocates extra pages for these vmcoreinfo_XXX variables,
>>> one advantage is that it enhances some safety of vmcoreinfo, because
>>> vmcoreinfo now is kept far away from other kernel data structures.
>> Can you preceed this patch with a patch that removes CRASHTIME from
>> vmcoreinfo?  If someone actually cares we can add a separate note that holds
>> a 64bit crashtime in the per cpu notes.  
> Hi Eric,
>
> Thanks for your review, I took some time and did some investigation.
>
> Removing "CRASHTIME=X" from vmcoreinfo_note will break user-space tools.
> For example, makedumpfile gets vmcoreinfo note information by reading
> "/sys/kernel/vmcoreinfo"  its PA, then get its "VA = PA | PAGE_OFFSET",
> and then get the timestamp. This operates in the first kernel even before
> kdump is loaded.

Think more, this is not a problem for "makedumpfile --mem-usage",
as the system doesn't have "CRASHTIME" before crash. But still we
may have the following concerns.

>
> Actually, even moving vmcoreinfo_note[] into the crash memory, it
> may have problems, for example, on s390 system the crash memory
> range will be unmapped, so I guess it may cause some risks.
>
> Additionally, there is no available way for us to allocate a page from the
> crash memory during kernel initialization, we only can achieve this during
> the kexec syscalls. There is not a neat way to implement a function to
> allocate pages from the crash memory during kernel initialization without
> some hack code added, because user-space tools(like kexec-tools) can
> allocate the crash segment by their own ways from the crash memory.
>
> That's why I only copy vmcoreinfo_data[] into the crash memory, and
> not touch vmcoreinfo_note, so vmcoreinfo_data is well protected in
> the crash memory copy, then in crash_save_vmcoreinfo(), we copy
> this guaranteed copy into vmcoreinfo_note[], so the correctness of
> vmcoreinfo_note[] is guaranteed. This is what [PATCH v3 3/3] does.
>
> The current crash_save_vmcoreinfo() only involves memory(memcpy)
> operations even for get_seconds(no locks), the only risk I can think
> of now is that vmcoreinfo_note pointer may be corrupted. If it is a concern,
> I guess we can put it into struct kimage" just like vmcoreinfo_XXX_copy
> in this patch. After all if kimage structure was corrupted when crash happens,
> we can do nothing but have to accept the fate.
>
> So does it really deserve to eliminate crash_save_vmcoreinfo()?
>
> Regards,
> Xunlei
>
>> As we are looking at reliability concerns removing CRASHTIME should make
>> everything in vmcoreinfo a boot time constant.  Which should simplify
>> everything considerably.
>>
>> Which means we only need to worry abou the per-cpu notes being written
>> at the time of a crash.
>>
>>> Suggested-by: Eric Biederman 
>>> Signed-off-by: Xunlei Pang 
>>> ---
>>>  arch/ia64/kernel/machine_kexec.c |  5 -
>>>  arch/x86/kernel/crash.c  |  2 +-
>>>  include/linux/kexec.h|  2 +-
>>>  kernel/kexec_core.c  | 29 -
>>>  kernel/ksysfs.c  |  2 +-
>>>  5 files changed, 27 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/arch/ia64/kernel/machine_kexec.c 
>>> b/arch/ia64/kernel/machine_kexec.c
>>> index 599507b..c14815d 100644
>>> --- a/arch/ia64/kernel/machine_kexec.c
>>> +++ b/arch/ia64/kernel/machine_kexec.c
>>> @@ -163,8 +163,3 @@ void arch_crash_save_vmcoreinfo(void)
>>>  #endif
>>>  }
>>>  
>>> -phys_addr_t paddr_vmcoreinfo_note(void)
>>> -{
>>> -   return ia64_tpa((unsigned long)(char *)_note);
>>> -}
>>> -
>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>> index 3741461..4d35fbb 100644
>>> --- a/arch/x86/kernel/crash.c
>>> +++ b/arch/x86/kernel/crash.c
>>> @@ -456,7 +456,7 @@ static int prepare_elf64_headers(struct crash_elf_data 
>>> *ced,
>>> bufp += sizeof(Elf64_Phdr);
>>> phdr->p_type = PT_NOTE;
>>> phdr->p_offset = phdr->p_paddr = paddr_vmcoreinfo_note();
>>> -   phdr->p_filesz = phdr->p_memsz = sizeof(vmcoreinfo_note);
>>> +   phdr->p_filesz = phdr->p_memsz = VMCOREINFO_NOTE_SIZE;
>>> (ehdr->e_phnum)++;
>>>  
>>>  #ifdef CONFIG_X86_64
>>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>>> index e98e546..f1c601b 100644
>>> --- a/include/linux/kexec.h
>>> +++ b/include/linux/kexec.h
>>> @@ 

Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Xunlei Pang]

On 03/22/2017 at 04:55 PM, Xunlei Pang wrote:
> On 03/21/2017 at 11:33 AM, Eric W. Biederman wrote:
>> Xunlei Pang  writes:
>>
>>> As Eric said,
>>> "what we need to do is move the variable vmcoreinfo_note out
>>> of the kernel's .bss section.  And modify the code to regenerate
>>> and keep this information in something like the control page.
>>>
>>> Definitely something like this needs a page all to itself, and ideally
>>> far away from any other kernel data structures.  I clearly was not
>>> watching closely the data someone decided to keep this silly thing
>>> in the kernel's .bss section."
>>>
>>> This patch allocates extra pages for these vmcoreinfo_XXX variables,
>>> one advantage is that it enhances some safety of vmcoreinfo, because
>>> vmcoreinfo now is kept far away from other kernel data structures.
>> Can you preceed this patch with a patch that removes CRASHTIME from
>> vmcoreinfo?  If someone actually cares we can add a separate note that holds
>> a 64bit crashtime in the per cpu notes.  
> Hi Eric,
>
> Thanks for your review, I took some time and did some investigation.
>
> Removing "CRASHTIME=X" from vmcoreinfo_note will break user-space tools.
> For example, makedumpfile gets vmcoreinfo note information by reading
> "/sys/kernel/vmcoreinfo"  its PA, then get its "VA = PA | PAGE_OFFSET",
> and then get the timestamp. This operates in the first kernel even before
> kdump is loaded.

Think more, this is not a problem for "makedumpfile --mem-usage",
as the system doesn't have "CRASHTIME" before crash. But still we
may have the following concerns.

>
> Actually, even moving vmcoreinfo_note[] into the crash memory, it
> may have problems, for example, on s390 system the crash memory
> range will be unmapped, so I guess it may cause some risks.
>
> Additionally, there is no available way for us to allocate a page from the
> crash memory during kernel initialization, we only can achieve this during
> the kexec syscalls. There is not a neat way to implement a function to
> allocate pages from the crash memory during kernel initialization without
> some hack code added, because user-space tools(like kexec-tools) can
> allocate the crash segment by their own ways from the crash memory.
>
> That's why I only copy vmcoreinfo_data[] into the crash memory, and
> not touch vmcoreinfo_note, so vmcoreinfo_data is well protected in
> the crash memory copy, then in crash_save_vmcoreinfo(), we copy
> this guaranteed copy into vmcoreinfo_note[], so the correctness of
> vmcoreinfo_note[] is guaranteed. This is what [PATCH v3 3/3] does.
>
> The current crash_save_vmcoreinfo() only involves memory(memcpy)
> operations even for get_seconds(no locks), the only risk I can think
> of now is that vmcoreinfo_note pointer may be corrupted. If it is a concern,
> I guess we can put it into struct kimage" just like vmcoreinfo_XXX_copy
> in this patch. After all if kimage structure was corrupted when crash happens,
> we can do nothing but have to accept the fate.
>
> So does it really deserve to eliminate crash_save_vmcoreinfo()?
>
> Regards,
> Xunlei
>
>> As we are looking at reliability concerns removing CRASHTIME should make
>> everything in vmcoreinfo a boot time constant.  Which should simplify
>> everything considerably.
>>
>> Which means we only need to worry abou the per-cpu notes being written
>> at the time of a crash.
>>
>>> Suggested-by: Eric Biederman 
>>> Signed-off-by: Xunlei Pang 
>>> ---
>>>  arch/ia64/kernel/machine_kexec.c |  5 -
>>>  arch/x86/kernel/crash.c  |  2 +-
>>>  include/linux/kexec.h|  2 +-
>>>  kernel/kexec_core.c  | 29 -
>>>  kernel/ksysfs.c  |  2 +-
>>>  5 files changed, 27 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/arch/ia64/kernel/machine_kexec.c 
>>> b/arch/ia64/kernel/machine_kexec.c
>>> index 599507b..c14815d 100644
>>> --- a/arch/ia64/kernel/machine_kexec.c
>>> +++ b/arch/ia64/kernel/machine_kexec.c
>>> @@ -163,8 +163,3 @@ void arch_crash_save_vmcoreinfo(void)
>>>  #endif
>>>  }
>>>  
>>> -phys_addr_t paddr_vmcoreinfo_note(void)
>>> -{
>>> -   return ia64_tpa((unsigned long)(char *)_note);
>>> -}
>>> -
>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>> index 3741461..4d35fbb 100644
>>> --- a/arch/x86/kernel/crash.c
>>> +++ b/arch/x86/kernel/crash.c
>>> @@ -456,7 +456,7 @@ static int prepare_elf64_headers(struct crash_elf_data 
>>> *ced,
>>> bufp += sizeof(Elf64_Phdr);
>>> phdr->p_type = PT_NOTE;
>>> phdr->p_offset = phdr->p_paddr = paddr_vmcoreinfo_note();
>>> -   phdr->p_filesz = phdr->p_memsz = sizeof(vmcoreinfo_note);
>>> +   phdr->p_filesz = phdr->p_memsz = VMCOREINFO_NOTE_SIZE;
>>> (ehdr->e_phnum)++;
>>>  
>>>  #ifdef CONFIG_X86_64
>>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>>> index e98e546..f1c601b 100644
>>> --- a/include/linux/kexec.h
>>> +++ b/include/linux/kexec.h
>>> @@ 

Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Xunlei Pang]

On 03/21/2017 at 11:33 AM, Eric W. Biederman wrote:
> Xunlei Pang  writes:
>
>> As Eric said,
>> "what we need to do is move the variable vmcoreinfo_note out
>> of the kernel's .bss section.  And modify the code to regenerate
>> and keep this information in something like the control page.
>>
>> Definitely something like this needs a page all to itself, and ideally
>> far away from any other kernel data structures.  I clearly was not
>> watching closely the data someone decided to keep this silly thing
>> in the kernel's .bss section."
>>
>> This patch allocates extra pages for these vmcoreinfo_XXX variables,
>> one advantage is that it enhances some safety of vmcoreinfo, because
>> vmcoreinfo now is kept far away from other kernel data structures.
> Can you preceed this patch with a patch that removes CRASHTIME from
> vmcoreinfo?  If someone actually cares we can add a separate note that holds
> a 64bit crashtime in the per cpu notes.  

Hi Eric,

Thanks for your review, I took some time and did some investigation.

Removing "CRASHTIME=X" from vmcoreinfo_note will break user-space tools.
For example, makedumpfile gets vmcoreinfo note information by reading
"/sys/kernel/vmcoreinfo"  its PA, then get its "VA = PA | PAGE_OFFSET",
and then get the timestamp. This operates in the first kernel even before
kdump is loaded.

Actually, even moving vmcoreinfo_note[] into the crash memory, it
may have problems, for example, on s390 system the crash memory
range will be unmapped, so I guess it may cause some risks.

Additionally, there is no available way for us to allocate a page from the
crash memory during kernel initialization, we only can achieve this during
the kexec syscalls. There is not a neat way to implement a function to
allocate pages from the crash memory during kernel initialization without
some hack code added, because user-space tools(like kexec-tools) can
allocate the crash segment by their own ways from the crash memory.

That's why I only copy vmcoreinfo_data[] into the crash memory, and
not touch vmcoreinfo_note, so vmcoreinfo_data is well protected in
the crash memory copy, then in crash_save_vmcoreinfo(), we copy
this guaranteed copy into vmcoreinfo_note[], so the correctness of
vmcoreinfo_note[] is guaranteed. This is what [PATCH v3 3/3] does.

The current crash_save_vmcoreinfo() only involves memory(memcpy)
operations even for get_seconds(no locks), the only risk I can think
of now is that vmcoreinfo_note pointer may be corrupted. If it is a concern,
I guess we can put it into struct kimage" just like vmcoreinfo_XXX_copy
in this patch. After all if kimage structure was corrupted when crash happens,
we can do nothing but have to accept the fate.

So does it really deserve to eliminate crash_save_vmcoreinfo()?

Regards,
Xunlei

>
> As we are looking at reliability concerns removing CRASHTIME should make
> everything in vmcoreinfo a boot time constant.  Which should simplify
> everything considerably.
>
> Which means we only need to worry abou the per-cpu notes being written
> at the time of a crash.
>
>> Suggested-by: Eric Biederman 
>> Signed-off-by: Xunlei Pang 
>> ---
>>  arch/ia64/kernel/machine_kexec.c |  5 -
>>  arch/x86/kernel/crash.c  |  2 +-
>>  include/linux/kexec.h|  2 +-
>>  kernel/kexec_core.c  | 29 -
>>  kernel/ksysfs.c  |  2 +-
>>  5 files changed, 27 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/ia64/kernel/machine_kexec.c 
>> b/arch/ia64/kernel/machine_kexec.c
>> index 599507b..c14815d 100644
>> --- a/arch/ia64/kernel/machine_kexec.c
>> +++ b/arch/ia64/kernel/machine_kexec.c
>> @@ -163,8 +163,3 @@ void arch_crash_save_vmcoreinfo(void)
>>  #endif
>>  }
>>  
>> -phys_addr_t paddr_vmcoreinfo_note(void)
>> -{
>> -return ia64_tpa((unsigned long)(char *)_note);
>> -}
>> -
>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>> index 3741461..4d35fbb 100644
>> --- a/arch/x86/kernel/crash.c
>> +++ b/arch/x86/kernel/crash.c
>> @@ -456,7 +456,7 @@ static int prepare_elf64_headers(struct crash_elf_data 
>> *ced,
>>  bufp += sizeof(Elf64_Phdr);
>>  phdr->p_type = PT_NOTE;
>>  phdr->p_offset = phdr->p_paddr = paddr_vmcoreinfo_note();
>> -phdr->p_filesz = phdr->p_memsz = sizeof(vmcoreinfo_note);
>> +phdr->p_filesz = phdr->p_memsz = VMCOREINFO_NOTE_SIZE;
>>  (ehdr->e_phnum)++;
>>  
>>  #ifdef CONFIG_X86_64
>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>> index e98e546..f1c601b 100644
>> --- a/include/linux/kexec.h
>> +++ b/include/linux/kexec.h
>> @@ -317,7 +317,7 @@ extern void *kexec_purgatory_get_symbol_addr(struct 
>> kimage *image,
>>  extern struct resource crashk_low_res;
>>  typedef u32 note_buf_t[KEXEC_NOTE_BYTES/4];
>>  extern note_buf_t __percpu *crash_notes;
>> -extern u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
>> +extern u32 *vmcoreinfo_note;
>>  extern size_t 

Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Dave Young]

On 03/21/17 at 10:18pm, Eric W. Biederman wrote:
> Dave Young  writes:
> 
> > On 03/20/17 at 10:33pm, Eric W. Biederman wrote:
> >> Xunlei Pang  writes:
> >> 
> >> > As Eric said,
> >> > "what we need to do is move the variable vmcoreinfo_note out
> >> > of the kernel's .bss section.  And modify the code to regenerate
> >> > and keep this information in something like the control page.
> >> >
> >> > Definitely something like this needs a page all to itself, and ideally
> >> > far away from any other kernel data structures.  I clearly was not
> >> > watching closely the data someone decided to keep this silly thing
> >> > in the kernel's .bss section."
> >> >
> >> > This patch allocates extra pages for these vmcoreinfo_XXX variables,
> >> > one advantage is that it enhances some safety of vmcoreinfo, because
> >> > vmcoreinfo now is kept far away from other kernel data structures.
> >> 
> >> Can you preceed this patch with a patch that removes CRASHTIME from
> >> vmcoreinfo?  If someone actually cares we can add a separate note that 
> >> holds
> >> a 64bit crashtime in the per cpu notes.  
> >
> > I think makedumpfile is using it, but I also vote to remove the
> > CRASHTIME. It is better not to do this while crashing and a makedumpfile
> > userspace patch is needed to drop the use of it.
> >
> >> 
> >> As we are looking at reliability concerns removing CRASHTIME should make
> >> everything in vmcoreinfo a boot time constant.  Which should simplify
> >> everything considerably.
> >
> > It is a nice improvement..
> 
> We also need to take a close look at what s390 is doing with vmcoreinfo.
> As apparently it is reading it in a different kind of crashdump process.

Yes, need careful review from s390 and maybe ppc64 especially about
patch 2/3, better to have comments from IBM about s390 dump tool and ppc
fadump. Added more cc.

Thanks
Dave

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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Eric W. Biederman]

Dave Young  writes:

> On 03/20/17 at 10:33pm, Eric W. Biederman wrote:
>> Xunlei Pang  writes:
>> 
>> > As Eric said,
>> > "what we need to do is move the variable vmcoreinfo_note out
>> > of the kernel's .bss section.  And modify the code to regenerate
>> > and keep this information in something like the control page.
>> >
>> > Definitely something like this needs a page all to itself, and ideally
>> > far away from any other kernel data structures.  I clearly was not
>> > watching closely the data someone decided to keep this silly thing
>> > in the kernel's .bss section."
>> >
>> > This patch allocates extra pages for these vmcoreinfo_XXX variables,
>> > one advantage is that it enhances some safety of vmcoreinfo, because
>> > vmcoreinfo now is kept far away from other kernel data structures.
>> 
>> Can you preceed this patch with a patch that removes CRASHTIME from
>> vmcoreinfo?  If someone actually cares we can add a separate note that holds
>> a 64bit crashtime in the per cpu notes.  
>
> I think makedumpfile is using it, but I also vote to remove the
> CRASHTIME. It is better not to do this while crashing and a makedumpfile
> userspace patch is needed to drop the use of it.
>
>> 
>> As we are looking at reliability concerns removing CRASHTIME should make
>> everything in vmcoreinfo a boot time constant.  Which should simplify
>> everything considerably.
>
> It is a nice improvement..

We also need to take a close look at what s390 is doing with vmcoreinfo.
As apparently it is reading it in a different kind of crashdump process.

Eric

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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Dave Young]

On 03/20/17 at 10:33pm, Eric W. Biederman wrote:
> Xunlei Pang  writes:
> 
> > As Eric said,
> > "what we need to do is move the variable vmcoreinfo_note out
> > of the kernel's .bss section.  And modify the code to regenerate
> > and keep this information in something like the control page.
> >
> > Definitely something like this needs a page all to itself, and ideally
> > far away from any other kernel data structures.  I clearly was not
> > watching closely the data someone decided to keep this silly thing
> > in the kernel's .bss section."
> >
> > This patch allocates extra pages for these vmcoreinfo_XXX variables,
> > one advantage is that it enhances some safety of vmcoreinfo, because
> > vmcoreinfo now is kept far away from other kernel data structures.
> 
> Can you preceed this patch with a patch that removes CRASHTIME from
> vmcoreinfo?  If someone actually cares we can add a separate note that holds
> a 64bit crashtime in the per cpu notes.  

I think makedumpfile is using it, but I also vote to remove the
CRASHTIME. It is better not to do this while crashing and a makedumpfile
userspace patch is needed to drop the use of it.

> 
> As we are looking at reliability concerns removing CRASHTIME should make
> everything in vmcoreinfo a boot time constant.  Which should simplify
> everything considerably.

It is a nice improvement..

> 
> Which means we only need to worry abou the per-cpu notes being written
> at the time of a crash.
> 
> > Suggested-by: Eric Biederman 
> > Signed-off-by: Xunlei Pang 
> > ---
> >  arch/ia64/kernel/machine_kexec.c |  5 -
> >  arch/x86/kernel/crash.c  |  2 +-
> >  include/linux/kexec.h|  2 +-
> >  kernel/kexec_core.c  | 29 -
> >  kernel/ksysfs.c  |  2 +-
> >  5 files changed, 27 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/ia64/kernel/machine_kexec.c 
> > b/arch/ia64/kernel/machine_kexec.c
> > index 599507b..c14815d 100644
> > --- a/arch/ia64/kernel/machine_kexec.c
> > +++ b/arch/ia64/kernel/machine_kexec.c
> > @@ -163,8 +163,3 @@ void arch_crash_save_vmcoreinfo(void)
> >  #endif
> >  }
> >  
> > -phys_addr_t paddr_vmcoreinfo_note(void)
> > -{
> > -   return ia64_tpa((unsigned long)(char *)_note);
> > -}
> > -
> > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > index 3741461..4d35fbb 100644
> > --- a/arch/x86/kernel/crash.c
> > +++ b/arch/x86/kernel/crash.c
> > @@ -456,7 +456,7 @@ static int prepare_elf64_headers(struct crash_elf_data 
> > *ced,
> > bufp += sizeof(Elf64_Phdr);
> > phdr->p_type = PT_NOTE;
> > phdr->p_offset = phdr->p_paddr = paddr_vmcoreinfo_note();
> > -   phdr->p_filesz = phdr->p_memsz = sizeof(vmcoreinfo_note);
> > +   phdr->p_filesz = phdr->p_memsz = VMCOREINFO_NOTE_SIZE;
> > (ehdr->e_phnum)++;
> >  
> >  #ifdef CONFIG_X86_64
> > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > index e98e546..f1c601b 100644
> > --- a/include/linux/kexec.h
> > +++ b/include/linux/kexec.h
> > @@ -317,7 +317,7 @@ extern void *kexec_purgatory_get_symbol_addr(struct 
> > kimage *image,
> >  extern struct resource crashk_low_res;
> >  typedef u32 note_buf_t[KEXEC_NOTE_BYTES/4];
> >  extern note_buf_t __percpu *crash_notes;
> > -extern u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
> > +extern u32 *vmcoreinfo_note;
> >  extern size_t vmcoreinfo_size;
> >  extern size_t vmcoreinfo_max_size;
> >  
> > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > index bfe62d5..e3a4bda 100644
> > --- a/kernel/kexec_core.c
> > +++ b/kernel/kexec_core.c
> > @@ -52,10 +52,10 @@
> >  note_buf_t __percpu *crash_notes;
> >  
> >  /* vmcoreinfo stuff */
> > -static unsigned char vmcoreinfo_data[VMCOREINFO_BYTES];
> > -u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
> > +static unsigned char *vmcoreinfo_data;
> >  size_t vmcoreinfo_size;
> > -size_t vmcoreinfo_max_size = sizeof(vmcoreinfo_data);
> > +size_t vmcoreinfo_max_size = VMCOREINFO_BYTES;
> > +u32 *vmcoreinfo_note;
> >  
> >  /* Flag to indicate we are going to kexec a new kernel */
> >  bool kexec_in_progress = false;
> > @@ -1369,6 +1369,9 @@ static void update_vmcoreinfo_note(void)
> >  
> >  void crash_save_vmcoreinfo(void)
> >  {
> > +   if (!vmcoreinfo_note)
> > +   return;
> > +
> > vmcoreinfo_append_str("CRASHTIME=%ld\n", get_seconds());
> > update_vmcoreinfo_note();
> >  }
> > @@ -1397,13 +1400,29 @@ void vmcoreinfo_append_str(const char *fmt, ...)
> >  void __weak arch_crash_save_vmcoreinfo(void)
> >  {}
> >  
> > -phys_addr_t __weak paddr_vmcoreinfo_note(void)
> > +phys_addr_t paddr_vmcoreinfo_note(void)
> >  {
> > -   return __pa_symbol((unsigned long)(char *)_note);
> > +   return __pa(vmcoreinfo_note);
> >  }
> >  
> >  static int __init crash_save_vmcoreinfo_init(void)
> >  {
> > +   /* One page should be enough for VMCOREINFO_BYTES under all archs */
> > +   vmcoreinfo_data = (unsigned char 

Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Petr Tesarik]

On Mon, 20 Mar 2017 13:50:31 +0800
Xunlei Pang  wrote:

> As Eric said,
> "what we need to do is move the variable vmcoreinfo_note out
> of the kernel's .bss section.  And modify the code to regenerate
> and keep this information in something like the control page.
> 
> Definitely something like this needs a page all to itself, and ideally
> far away from any other kernel data structures.  I clearly was not
> watching closely the data someone decided to keep this silly thing
> in the kernel's .bss section."
> 
> This patch allocates extra pages for these vmcoreinfo_XXX variables,
> one advantage is that it enhances some safety of vmcoreinfo, because
> vmcoreinfo now is kept far away from other kernel data structures.

Yes, I like this patch set very much now. Thank you!

Petr T

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Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Eric W. Biederman]

Xunlei Pang  writes:

> As Eric said,
> "what we need to do is move the variable vmcoreinfo_note out
> of the kernel's .bss section.  And modify the code to regenerate
> and keep this information in something like the control page.
>
> Definitely something like this needs a page all to itself, and ideally
> far away from any other kernel data structures.  I clearly was not
> watching closely the data someone decided to keep this silly thing
> in the kernel's .bss section."
>
> This patch allocates extra pages for these vmcoreinfo_XXX variables,
> one advantage is that it enhances some safety of vmcoreinfo, because
> vmcoreinfo now is kept far away from other kernel data structures.

Can you preceed this patch with a patch that removes CRASHTIME from
vmcoreinfo?  If someone actually cares we can add a separate note that holds
a 64bit crashtime in the per cpu notes.  

As we are looking at reliability concerns removing CRASHTIME should make
everything in vmcoreinfo a boot time constant.  Which should simplify
everything considerably.

Which means we only need to worry abou the per-cpu notes being written
at the time of a crash.

> Suggested-by: Eric Biederman 
> Signed-off-by: Xunlei Pang 
> ---
>  arch/ia64/kernel/machine_kexec.c |  5 -
>  arch/x86/kernel/crash.c  |  2 +-
>  include/linux/kexec.h|  2 +-
>  kernel/kexec_core.c  | 29 -
>  kernel/ksysfs.c  |  2 +-
>  5 files changed, 27 insertions(+), 13 deletions(-)
>
> diff --git a/arch/ia64/kernel/machine_kexec.c 
> b/arch/ia64/kernel/machine_kexec.c
> index 599507b..c14815d 100644
> --- a/arch/ia64/kernel/machine_kexec.c
> +++ b/arch/ia64/kernel/machine_kexec.c
> @@ -163,8 +163,3 @@ void arch_crash_save_vmcoreinfo(void)
>  #endif
>  }
>  
> -phys_addr_t paddr_vmcoreinfo_note(void)
> -{
> - return ia64_tpa((unsigned long)(char *)_note);
> -}
> -
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 3741461..4d35fbb 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -456,7 +456,7 @@ static int prepare_elf64_headers(struct crash_elf_data 
> *ced,
>   bufp += sizeof(Elf64_Phdr);
>   phdr->p_type = PT_NOTE;
>   phdr->p_offset = phdr->p_paddr = paddr_vmcoreinfo_note();
> - phdr->p_filesz = phdr->p_memsz = sizeof(vmcoreinfo_note);
> + phdr->p_filesz = phdr->p_memsz = VMCOREINFO_NOTE_SIZE;
>   (ehdr->e_phnum)++;
>  
>  #ifdef CONFIG_X86_64
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index e98e546..f1c601b 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -317,7 +317,7 @@ extern void *kexec_purgatory_get_symbol_addr(struct 
> kimage *image,
>  extern struct resource crashk_low_res;
>  typedef u32 note_buf_t[KEXEC_NOTE_BYTES/4];
>  extern note_buf_t __percpu *crash_notes;
> -extern u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
> +extern u32 *vmcoreinfo_note;
>  extern size_t vmcoreinfo_size;
>  extern size_t vmcoreinfo_max_size;
>  
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index bfe62d5..e3a4bda 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -52,10 +52,10 @@
>  note_buf_t __percpu *crash_notes;
>  
>  /* vmcoreinfo stuff */
> -static unsigned char vmcoreinfo_data[VMCOREINFO_BYTES];
> -u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
> +static unsigned char *vmcoreinfo_data;
>  size_t vmcoreinfo_size;
> -size_t vmcoreinfo_max_size = sizeof(vmcoreinfo_data);
> +size_t vmcoreinfo_max_size = VMCOREINFO_BYTES;
> +u32 *vmcoreinfo_note;
>  
>  /* Flag to indicate we are going to kexec a new kernel */
>  bool kexec_in_progress = false;
> @@ -1369,6 +1369,9 @@ static void update_vmcoreinfo_note(void)
>  
>  void crash_save_vmcoreinfo(void)
>  {
> + if (!vmcoreinfo_note)
> + return;
> +
>   vmcoreinfo_append_str("CRASHTIME=%ld\n", get_seconds());
>   update_vmcoreinfo_note();
>  }
> @@ -1397,13 +1400,29 @@ void vmcoreinfo_append_str(const char *fmt, ...)
>  void __weak arch_crash_save_vmcoreinfo(void)
>  {}
>  
> -phys_addr_t __weak paddr_vmcoreinfo_note(void)
> +phys_addr_t paddr_vmcoreinfo_note(void)
>  {
> - return __pa_symbol((unsigned long)(char *)_note);
> + return __pa(vmcoreinfo_note);
>  }
>  
>  static int __init crash_save_vmcoreinfo_init(void)
>  {
> + /* One page should be enough for VMCOREINFO_BYTES under all archs */
> + vmcoreinfo_data = (unsigned char *)get_zeroed_page(GFP_KERNEL);
> + if (!vmcoreinfo_data) {
> + pr_warn("Memory allocation for vmcoreinfo_data failed\n");
> + return -ENOMEM;
> + }
> +
> + vmcoreinfo_note = alloc_pages_exact(VMCOREINFO_NOTE_SIZE,
> + GFP_KERNEL | __GFP_ZERO);
> + if (!vmcoreinfo_note) {
> + free_page((unsigned long)vmcoreinfo_data);
> + vmcoreinfo_data = NULL;
> + pr_warn("Memory 

[PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Xunlei Pang]

As Eric said,
"what we need to do is move the variable vmcoreinfo_note out
of the kernel's .bss section.  And modify the code to regenerate
and keep this information in something like the control page.

Definitely something like this needs a page all to itself, and ideally
far away from any other kernel data structures.  I clearly was not
watching closely the data someone decided to keep this silly thing
in the kernel's .bss section."

This patch allocates extra pages for these vmcoreinfo_XXX variables,
one advantage is that it enhances some safety of vmcoreinfo, because
vmcoreinfo now is kept far away from other kernel data structures.

Suggested-by: Eric Biederman 
Signed-off-by: Xunlei Pang 
---
 arch/ia64/kernel/machine_kexec.c |  5 -
 arch/x86/kernel/crash.c  |  2 +-
 include/linux/kexec.h|  2 +-
 kernel/kexec_core.c  | 29 -
 kernel/ksysfs.c  |  2 +-
 5 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/arch/ia64/kernel/machine_kexec.c b/arch/ia64/kernel/machine_kexec.c
index 599507b..c14815d 100644
--- a/arch/ia64/kernel/machine_kexec.c
+++ b/arch/ia64/kernel/machine_kexec.c
@@ -163,8 +163,3 @@ void arch_crash_save_vmcoreinfo(void)
 #endif
 }
 
-phys_addr_t paddr_vmcoreinfo_note(void)
-{
-   return ia64_tpa((unsigned long)(char *)_note);
-}
-
diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 3741461..4d35fbb 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -456,7 +456,7 @@ static int prepare_elf64_headers(struct crash_elf_data *ced,
bufp += sizeof(Elf64_Phdr);
phdr->p_type = PT_NOTE;
phdr->p_offset = phdr->p_paddr = paddr_vmcoreinfo_note();
-   phdr->p_filesz = phdr->p_memsz = sizeof(vmcoreinfo_note);
+   phdr->p_filesz = phdr->p_memsz = VMCOREINFO_NOTE_SIZE;
(ehdr->e_phnum)++;
 
 #ifdef CONFIG_X86_64
diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index e98e546..f1c601b 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -317,7 +317,7 @@ extern void *kexec_purgatory_get_symbol_addr(struct kimage 
*image,
 extern struct resource crashk_low_res;
 typedef u32 note_buf_t[KEXEC_NOTE_BYTES/4];
 extern note_buf_t __percpu *crash_notes;
-extern u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
+extern u32 *vmcoreinfo_note;
 extern size_t vmcoreinfo_size;
 extern size_t vmcoreinfo_max_size;
 
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index bfe62d5..e3a4bda 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -52,10 +52,10 @@
 note_buf_t __percpu *crash_notes;
 
 /* vmcoreinfo stuff */
-static unsigned char vmcoreinfo_data[VMCOREINFO_BYTES];
-u32 vmcoreinfo_note[VMCOREINFO_NOTE_SIZE/4];
+static unsigned char *vmcoreinfo_data;
 size_t vmcoreinfo_size;
-size_t vmcoreinfo_max_size = sizeof(vmcoreinfo_data);
+size_t vmcoreinfo_max_size = VMCOREINFO_BYTES;
+u32 *vmcoreinfo_note;
 
 /* Flag to indicate we are going to kexec a new kernel */
 bool kexec_in_progress = false;
@@ -1369,6 +1369,9 @@ static void update_vmcoreinfo_note(void)
 
 void crash_save_vmcoreinfo(void)
 {
+   if (!vmcoreinfo_note)
+   return;
+
vmcoreinfo_append_str("CRASHTIME=%ld\n", get_seconds());
update_vmcoreinfo_note();
 }
@@ -1397,13 +1400,29 @@ void vmcoreinfo_append_str(const char *fmt, ...)
 void __weak arch_crash_save_vmcoreinfo(void)
 {}
 
-phys_addr_t __weak paddr_vmcoreinfo_note(void)
+phys_addr_t paddr_vmcoreinfo_note(void)
 {
-   return __pa_symbol((unsigned long)(char *)_note);
+   return __pa(vmcoreinfo_note);
 }
 
 static int __init crash_save_vmcoreinfo_init(void)
 {
+   /* One page should be enough for VMCOREINFO_BYTES under all archs */
+   vmcoreinfo_data = (unsigned char *)get_zeroed_page(GFP_KERNEL);
+   if (!vmcoreinfo_data) {
+   pr_warn("Memory allocation for vmcoreinfo_data failed\n");
+   return -ENOMEM;
+   }
+
+   vmcoreinfo_note = alloc_pages_exact(VMCOREINFO_NOTE_SIZE,
+   GFP_KERNEL | __GFP_ZERO);
+   if (!vmcoreinfo_note) {
+   free_page((unsigned long)vmcoreinfo_data);
+   vmcoreinfo_data = NULL;
+   pr_warn("Memory allocation for vmcoreinfo_note failed\n");
+   return -ENOMEM;
+   }
+
VMCOREINFO_OSRELEASE(init_uts_ns.name.release);
VMCOREINFO_PAGESIZE(PAGE_SIZE);
 
diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index ee1bc1b..9de6fcc 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -130,7 +130,7 @@ static ssize_t vmcoreinfo_show(struct kobject *kobj,
 {
phys_addr_t vmcore_base = paddr_vmcoreinfo_note();
return sprintf(buf, "%pa %x\n", _base,
-  (unsigned int)sizeof(vmcoreinfo_note));
+   (unsigned int)VMCOREINFO_NOTE_SIZE);
 }
 KERNEL_ATTR_RO(vmcoreinfo);
 
-- 
1.8.3.1