Re: [PATCH v2 1/2] x86/mm/ident_map: Add PUD level 1GB page support

[Xunlei Pang]

On 04/26/2017 at 04:09 PM, Ingo Molnar wrote:
> * Xunlei Pang <xlpang@xxxxxxxxxx> wrote:
>
>> The current kernel_ident_mapping_init() creates the identity
>> mapping using 2MB page(PMD level), this patch adds the 1GB
>> page(PUD level) support.
>>
>> This is useful on large machines to save some reserved memory
>> (as paging structures) in the kdump case when kexec setups up
>> identity mappings before booting into the new kernel.
>>
>> We will utilize this new support in the following patch.
> Well, the primary advantage would be better TLB coverage/performance, because we'd 
> utilize 1GB TLBs instead of 2MB ones, right?
>
> Any kexec fallout is secondary.
>
> And I'd like to hear more about the primary advantage: what are the effects of 
> this change on a typical test system you have access to:
>
> - For example what percentage of the identity mapping was 4K mapped (if any) and
>   2MB mapped - and how did this change due to the patch - how many 2MB mappings
>   remained and how many 1GB mappings were added?
>
> - Is there anything else we could do to improve the in-RAM layout of kernel data 
>   structures. For example IIRC the CPU breaks up all TLBs under 2MB physical into 
>   4K TLBs. Is this the current limit and could we just reserve all that space and 
>   not use it for anything important? 2MB of RAM wasted is a very small amount of 
>   space, compared to the potential performance advantages.

Currently kernel_ident_mapping_init() only setups PMD large pages, seems there are no 4KB
mapped or others. Do you mean init_memory_mapping()->kernel_physical_mapping_init()?

The new struct x86_mapping_info::direct_gbpages added has the default value false, it has no
effect on the existing code, after patch 2, there will be 1GB pages(1GB TLBs) for kexec ident
mapping if direct_gbpages is true.

>
>>  	void *(*alloc_pgt_page)(void *); /* allocate buf for page table */
>>  	void *context;			 /* context for alloc_pgt_page */
>> -	unsigned long pmd_flag;		 /* page flag for PMD entry */
>> +	unsigned long page_flag;	 /* page flag for PMD or PUD entry */
>>  	unsigned long offset;		 /* ident mapping offset */
>> +	bool direct_gbpages;		/* PUD level 1GB page support */
> Doesn't follow the existing alignment.

Ah, yes, missed the space. Thanks!

Regards,
Xunlei