On 1/12/22 11:43 AM, Kirill A. Shutemov wrote:
> On Tue, Jan 11, 2022 at 11:10:40AM -0800, Dave Hansen wrote:
>> On 1/11/22 03:33, Kirill A. Shutemov wrote:
>>> Unaccepted memory bitmap is allocated during decompression stage and
>>> handed over to main kernel image via boot_params. The bitmap is used to
>>> track if memory has been accepted.
>>>
>>> Reserve unaccepted memory bitmap has to prevent reallocating memory for
>>> other means.
>>
>> I'm having a hard time parsing that changelog, especially the second
>> paragraph. Could you give it another shot?
>
> What about this:
>
> Unaccepted memory bitmap is allocated during decompression stage and
> handed over to main kernel image via boot_params.
>
> Kernel tracks what memory has been accepted in the bitmap.
>
> Reserve memory where the bitmap is placed to prevent memblock from
> re-allocating the memory for other needs.
>
> ?
Ahh, I get what you're trying to say now. But, it still really lacks a
coherent problem statement. How about this?
== Problem ==
A given page of memory can only be accepted once. The kernel
has a need to accept memory both in the early decompression
stage and during normal runtime.
== Solution ==
Use a bitmap to communicate the acceptance state of each page
between the decompression stage and normal runtime. This
eliminates the possibility of attempting to double-accept a
page.
== Details ==
Allocate the bitmap during decompression stage and hand it over
to the main kernel image via boot_params.
In the runtime kernel, reserve the bitmap's memory to ensure
nothing overwrites it.
>>> + /* Mark unaccepted memory bitmap reserved */
>>> + if (boot_params.unaccepted_memory) {
>>> + unsigned long size;
>>> +
>>> + /* One bit per 2MB */
>>> + size = DIV_ROUND_UP(e820__end_of_ram_pfn() * PAGE_SIZE,
>>> + PMD_SIZE * BITS_PER_BYTE);
>>> + memblock_reserve(boot_params.unaccepted_memory, size);
>>> + }
>>
>> Is it OK that the size of the bitmap is inferred from
>> e820__end_of_ram_pfn()? Is this OK in the presence of mem= and other things
>> that muck with the e820?
>
> Good question. I think we are fine. If kernel is not able to allocate
> memory from a part of physical address space we don't need the bitmap for
> it either.
That's a good point. If the e820 range does a one-way shrink it's
probably fine. The only problem would be if the bitmap had space for
for stuff past e820__end_of_ram_pfn() *and* it later needed to be accepted.
Would it be worth recording the size of the reservation and then
double-checking against it in the bitmap operations?
