Re: [PATCHv2 5/7] x86/mm: Reserve unaccepted memory bitmap

[Dave Hansen]

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?