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

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Date:   Fri, 5 May 2017 08:52:53 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Xunlei Pang <xlpang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        akpm@...ux-foundation.org, Eric Biederman <ebiederm@...ssion.com>,
        Dave Young <dyoung@...hat.com>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yinghai Lu <yinghai@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity
 mapping if available

* Xunlei Pang <xlpang@...hat.com> 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.

Thanks,

	Ingo

============================>

Author: Xunlei Pang <xlpang@...hat.com>

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. )