Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity mapping if available

From: Ingo Molnar
Date: Fri May 05 2017 - 02:53:03 EST

* Xunlei Pang <xlpang@xxxxxxxxxx> wrote:

> @@ -122,6 +122,10 @@ static int init_pgtable(struct kimage *image, unsigned long start_pgtable)
> level4p = (pgd_t *)__va(start_pgtable);
> clear_page(level4p);
> +
> + if (direct_gbpages)
> + info.direct_gbpages = true;

No, this should be keyed off the CPU feature (X86_FEATURE_GBPAGES) automatically,
not set blindly! AFAICS this patch will crash kexec on any CPU that does not
support gbpages.

I only noticed this problem after having fixed/enhanced all the changelogs - so
please pick up the new changelog up from the log below.




Author: Xunlei Pang <xlpang@xxxxxxxxxx>

x86/mm: Add support for gbpages to kernel_ident_mapping_init()

Kernel identity mappings on x86-64 kernels are created in two
ways: by the early x86 boot code, or by kernel_ident_mapping_init().

Native kernels (which is the dominant usecase) use the former,
but the kexec and the hibernation code uses kernel_ident_mapping_init().

There's a subtle difference between these two ways of how identity
mappings are created, the current kernel_ident_mapping_init() code
creates identity mappings always using 2MB page(PMD level) - while
the native kernel boot path also utilizes gbpages where available.

This difference is suboptimal both for performance and for memory
usage: kernel_ident_mapping_init() needs to allocate pages for the
page tables when creating the new identity mappings.

This patch adds 1GB page(PUD level) support to kernel_ident_mapping_init()
to address these concerns.

The primary advantage would be better TLB coverage/performance,
because we'd utilize 1GB TLBs instead of 2MB ones.

It is also useful for machines with large number of memory to
save paging structure allocations(around 4MB/TB using 2MB page)
when setting identity mappings for all the memory, after using
1GB page it will consume only 8KB/TB.

( Note that this change alone does not activate gbpages in kexec,
we are doing that in a separate patch. )