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powerpc/mm: Add comments to the vmemmap layout

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Add some explaination to the layout of vmemmap virtual address
space and how physical page mapping is only used for valid PFNs
present at any point on the system.

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Anshuman Khandual 2017-04-06 19:44:49 +05:30 committed by Michael Ellerman
parent c642af9c41
commit b0f36c10de
1 changed files with 75 additions and 0 deletions
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75
arch/powerpc/mm/pgtable-hash64.c
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@ -21,6 +21,81 @@
#include <trace/events/thp.h>
#ifdef CONFIG_SPARSEMEM_VMEMMAP
/*
* vmemmap is the starting address of the virtual address space where
* struct pages are allocated for all possible PFNs present on the system
* including holes and bad memory (hence sparse). These virtual struct
* pages are stored in sequence in this virtual address space irrespective
* of the fact whether the corresponding PFN is valid or not. This achieves
* constant relationship between address of struct page and its PFN.
*
* During boot or memory hotplug operation when a new memory section is
* added, physical memory allocation (including hash table bolting) will
* be performed for the set of struct pages which are part of the memory
* section. This saves memory by not allocating struct pages for PFNs
* which are not valid.
*
* ----------------------------------------------
* | PHYSICAL ALLOCATION OF VIRTUAL STRUCT PAGES|
* ----------------------------------------------
*
* f000000000000000 c000000000000000
* vmemmap +--------------+ +--------------+
* + | page struct | +--------------> | page struct |
* | +--------------+ +--------------+
* | | page struct | +--------------> | page struct |
* | +--------------+ | +--------------+
* | | page struct | + +------> | page struct |
* | +--------------+ | +--------------+
* | | page struct | | +--> | page struct |
* | +--------------+ | | +--------------+
* | | page struct | | |
* | +--------------+ | |
* | | page struct | | |
* | +--------------+ | |
* | | page struct | | |
* | +--------------+ | |
* | | page struct | | |
* | +--------------+ | |
* | | page struct | +-------+ |
* | +--------------+ |
* | | page struct | +-----------+
* | +--------------+
* | | page struct | No mapping
* | +--------------+
* | | page struct | No mapping
* v +--------------+
*
* -----------------------------------------
* | RELATION BETWEEN STRUCT PAGES AND PFNS|
* -----------------------------------------
*
* vmemmap +--------------+ +---------------+
* + | page struct | +-------------> | PFN |
* | +--------------+ +---------------+
* | | page struct | +-------------> | PFN |
* | +--------------+ +---------------+
* | | page struct | +-------------> | PFN |
* | +--------------+ +---------------+
* | | page struct | +-------------> | PFN |
* | +--------------+ +---------------+
* | | |
* | +--------------+
* | | |
* | +--------------+
* | | |
* | +--------------+ +---------------+
* | | page struct | +-------------> | PFN |
* | +--------------+ +---------------+
* | | |
* | +--------------+
* | | |
* | +--------------+ +---------------+
* | | page struct | +-------------> | PFN |
* | +--------------+ +---------------+
* | | page struct | +-------------> | PFN |
* v +--------------+ +---------------+
*/
/*
* On hash-based CPUs, the vmemmap is bolted in the hash table.
*