[PATCH v2 1/2] x86/mm/ident_map: Add PUD level 1GB page support
xpang at redhat.com
Wed Apr 26 01:56:33 PDT 2017
On 04/26/2017 at 04:09 PM, Ingo Molnar wrote:
> * Xunlei Pang <xlpang at redhat.com> 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!
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