memblock_reserve or memblock_remove to reserve a page

[Min-Hua Chen]

On Wed, Sep 14, 2016 at 3:17 PM, Nikhil Utane <nikhil.subscribed at gmail.com>
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 gmail.com> 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 gmail.com
>> > 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
>>>
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>>>
>>
>
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