Re: [PATCHv4 5/8] x86/mm: Reserve unaccepted memory bitmap

Dave Hansen
2022-04-08T18:08:47Z

On 4/5/22 16:43, Kirill A. Shutemov wrote:
> 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.
>
> A bitmap used 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.

... which is fatal, right? Could you include that an also the rationale
for why it is fatal?

> The bitmap is allocated in EFI stub, decompression stage updates the state
> of pages used for the kernel and initrd and hands the bitmap over to the
> main kernel image via boot_params.

This is really good info. Could we maybe expand it a bit?

There are several steps in the bitmap's lifecycle:
1. Bitmap is allocated in the EFI stub
2. The kernel decompression code locates it, accepts some pages before
decompression and marks them in the bitmap.
3. Decompression code points to the bitmap via a boot_param
4. Main kernel locates bitmap via the boot_param and uses it to guide
runtime acceptance decisions.

> In the runtime kernel, reserve the bitmap's memory to ensure nothing
> overwrites it.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
> arch/x86/kernel/e820.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index f267205f2d5a..22d1fe48dcba 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -1316,6 +1316,16 @@ void __init e820__memblock_setup(void)
> int i;
> u64 end;
>
> + /* 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);
> + }

One oddity here: The size is implied by the e820's contents. Did you
mention somewhere that unaccepted memory is considered E820_TYPE_RAM?
It *has* to be in order for e820__end_of_ram_pfn() to work, right?