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From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
To: linuxppc-dev@lists.ozlabs.org
Cc: aneesh.kumar@linux.vnet.ibm.com, mpe@ellerman.id.au
Subject: [PATCH 2/3] powerpc/mm: Add comments to the vmemmap layout
Date: Thu,  6 Apr 2017 19:44:49 +0530	[thread overview]
Message-ID: <20170406141450.16060-2-khandual@linux.vnet.ibm.com> (raw)
In-Reply-To: <20170406141450.16060-1-khandual@linux.vnet.ibm.com>

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>
---
Previous discussions around this http://patchwork.ozlabs.org/patch/584111/

 arch/powerpc/mm/pgtable-hash64.c | 75 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/arch/powerpc/mm/pgtable-hash64.c b/arch/powerpc/mm/pgtable-hash64.c
index 8b85a14..16877f6 100644
--- a/arch/powerpc/mm/pgtable-hash64.c
+++ b/arch/powerpc/mm/pgtable-hash64.c
@@ -22,6 +22,81 @@
 
 #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.
  *
  */
-- 
1.9.3

  reply	other threads:[~2017-04-06 14:15 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-06 14:14 [PATCH 1/3] powerpc/mm: Rename variable to reflect start address of a section Anshuman Khandual
2017-04-06 14:14 ` Anshuman Khandual [this message]
2017-06-29 12:21   ` [2/3] powerpc/mm: Add comments to the vmemmap layout Michael Ellerman
2017-04-06 14:14 ` [PATCH 3/3] powerpc/mm: Add comments on vmemmap physical mapping Anshuman Khandual
2017-06-29 12:21   ` [3/3] " Michael Ellerman
2017-04-14 13:37 ` [PATCH 1/3] powerpc/mm: Rename variable to reflect start address of a section Anshuman Khandual

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