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! Regards, Xunlei
