memblock_reserve or memblock_remove to reserve a page

Min-Hua Chen orca.chen at
Thu Sep 15 19:22:12 EDT 2016

On Thu, Sep 15, 2016 at 4:23 PM, Nikhil Utane <nikhil.subscribed at>

> MH,
> Let me give a bit of background of the issue.
> We are facing an issue where 4 bytes of physical memory is getting
> corrupted (set to 0) at a fixed offset.
> This offset is always fixed 0x00A4DDC0 (PFN: 0xA4D). The problem manifests
> in form of SIGILL for some random user-space application where its text
> area is corrupted. At this moment we are not able to identify who is
> causing the corruption. While we continue to investigate that (no HW
> breakpoint support :(), I thought we could at least mask the problem since
> we know the corruption is always occurring at a fixed offset.
> Therefore we want to reserve the memory so that kernel does not give it to
> anyone.
> We tried passing it via kernel command-line parameter (using memblock) but
> did not see it working. Finally we modified the function
> early_reserve_mem_dt() in file "linux-3.12.19/arch/powerpc/kernel/prom.c"
> to directly reserve the memory.
> base1 = 0xA4D000; size1=0x1000;
> memblock_reserve(base1, size1);
> To check if reservation is working and to monitor the corruption we wrote
> a kernel module that does a ioremap to page 0xA4D. We then poison it with
> fixed data. What we found was that, in few runs, this memory was intact and
> in few others it would change. We tried both memblock_reserve() as well as
> memblock_remove(). Unfortunately we continue to get the SIGILL at the same
> offset.
> Is there any other way to block a physical memory page?
> ioremap code (relevant lines):
> static char* sigill_mon_addr;
> #define ADDR_TEST 0xA4D00
> sigill_mon_addr = (char*)ioremap(ADDR_TEST, 4096);
> -Thanks
> Nikhil
Hi Nikhil,

How can a reserved page or reserved page with no-map causes a SIGILL? The
reserved page should not be
allocated to other users. Your applications should be fine even the
corruption remains on the reserved page.

I think you have to make sure the page is reserved reserved on your system.
For a memblock_remove() pages. Write a simple function to perform a CPU
write through the linear mapping address.
You should get a data abort and make sure that no CPU can access the page.
If the corruption remains, the corruption
may be caused by other H/W modules.


> On Thu, Sep 15, 2016 at 5:35 AM, Min-Hua Chen <orca.chen at> wrote:
>> On Wed, Sep 14, 2016 at 3:17 PM, Nikhil Utane <
>> nikhil.subscribed at> wrote:
>>> Thank You MH Chen for your response.
>>> So does that mean with memblock_reserve(), a kernel module can call
>>> phys_to_virt(), create a linear mapping and modify that memory?
>>> Where as with memblock_remove(), a kernel module can call ioremap() and
>>> then modify the memory?
>> Not really. It depends on the wether the reserved memory is in a linear
>> mapping range. For example, arm32 only creates linear mapping
>> within 1GB range because arm32 has only 1GB of kernel space virtual
>> memory. arm64 creates linear mapping for a large range
>> of memory (depends on ARM64_VA_BITS_xx).
>> for memblock_remove() memory, You can use ioremap() to access the memory.
>>> What would explain that only in some runs the memory is modified and in
>>> some runs it is not (for both the functions)? Shouldn't this
>>> reserved/removed memory never be modified unless someone is directly trying
>>> to write to that specific page?
>> They should not be modified. How do you write to the reserved memory? Can
>> you post the source code?
>> -MH
>>> -Regards
>>> Nikhil
>>> On Sun, Sep 11, 2016 at 6:08 AM, Min-Hua Chen <orca.chen at>
>>> wrote:
>>>> Hi Nikhil,
>>>> memblock_reserve() adds a given memory to the "memblock.reserved" list,
>>>> it ends up to mark the given range of pages as "reserved". It means the
>>>> pages are reserved and will not be allocated to other users. The kernel
>>>> still can see the pages, create linear mappings on them, even access them
>>>> by linear mappings.
>>>> memblock_remove() removes a given memory from the "memblock.memory"
>>>> list, it ends to removed from kernel's memory management system. The memory
>>>> will not have page structure, no linear mapping on them. It prevents the
>>>> memory from CPU accessing by the linear address. To access the memory (by
>>>> CPU), you must use ioremap() to create a mapping to them.
>>>> MH Chen
>>>> On Fri, Sep 9, 2016 at 5:29 PM, Nikhil Utane <
>>>> nikhil.subscribed at> wrote:
>>>>> Hi,
>>>>> I want to reserve a physical memory page with a fixed PFN. I do not
>>>>> want this page to be used by anyone else. I am calling memblock_reserve()
>>>>> to supposedly reserve the page. I am writing some content into this page.
>>>>> What I see is that during some runs the content of this page is modified
>>>>> (either fully or sometimes partially). In few runs, I see it as intact. Is
>>>>> it expected that even after calling memblock_reserve() the kernel can
>>>>> allocate this physical page for any other purpose? How is memblock_remove()
>>>>> different from memblock_reserve? I tried reading up but didn't see any
>>>>> useful information. What I understood is memblock_remove will completely
>>>>> remove from kernel's allocation mechanism. Should I then be using remove
>>>>> instead of reserve?
>>>>> -Thanks
>>>>> Nikhil
>>>>> _______________________________________________
>>>>> Kernelnewbies mailing list
>>>>> Kernelnewbies at
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