memblock_reserve or memblock_remove to reserve a page

[nikhil.subscribed@gmail.com (Nikhil Utane)]

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?

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?

-Regards
Nikhil

On Sun, Sep 11, 2016 at 6:08 AM, Min-Hua Chen <orca.chen@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@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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