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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:05 EST.

1	
2				     ====================
3				     HIGH MEMORY HANDLING
4				     ====================
5	
6	By: Peter Zijlstra <a.p.zijlstra@chello.nl>
7	
8	Contents:
9	
10	 (*) What is high memory?
11	
12	 (*) Temporary virtual mappings.
13	
14	 (*) Using kmap_atomic.
15	
16	 (*) Cost of temporary mappings.
17	
18	 (*) i386 PAE.
19	
20	
21	====================
22	WHAT IS HIGH MEMORY?
23	====================
24	
25	High memory (highmem) is used when the size of physical memory approaches or
26	exceeds the maximum size of virtual memory.  At that point it becomes
27	impossible for the kernel to keep all of the available physical memory mapped
28	at all times.  This means the kernel needs to start using temporary mappings of
29	the pieces of physical memory that it wants to access.
30	
31	The part of (physical) memory not covered by a permanent mapping is what we
32	refer to as 'highmem'.  There are various architecture dependent constraints on
33	where exactly that border lies.
34	
35	In the i386 arch, for example, we choose to map the kernel into every process's
36	VM space so that we don't have to pay the full TLB invalidation costs for
37	kernel entry/exit.  This means the available virtual memory space (4GiB on
38	i386) has to be divided between user and kernel space.
39	
40	The traditional split for architectures using this approach is 3:1, 3GiB for
41	userspace and the top 1GiB for kernel space:
42	
43			+--------+ 0xffffffff
44			| Kernel |
45			+--------+ 0xc0000000
46			|        |
47			| User   |
48			|        |
49			+--------+ 0x00000000
50	
51	This means that the kernel can at most map 1GiB of physical memory at any one
52	time, but because we need virtual address space for other things - including
53	temporary maps to access the rest of the physical memory - the actual direct
54	map will typically be less (usually around ~896MiB).
55	
56	Other architectures that have mm context tagged TLBs can have separate kernel
57	and user maps.  Some hardware (like some ARMs), however, have limited virtual
58	space when they use mm context tags.
59	
60	
61	==========================
62	TEMPORARY VIRTUAL MAPPINGS
63	==========================
64	
65	The kernel contains several ways of creating temporary mappings:
66	
67	 (*) vmap().  This can be used to make a long duration mapping of multiple
68	     physical pages into a contiguous virtual space.  It needs global
69	     synchronization to unmap.
70	
71	 (*) kmap().  This permits a short duration mapping of a single page.  It needs
72	     global synchronization, but is amortized somewhat.  It is also prone to
73	     deadlocks when using in a nested fashion, and so it is not recommended for
74	     new code.
75	
76	 (*) kmap_atomic().  This permits a very short duration mapping of a single
77	     page.  Since the mapping is restricted to the CPU that issued it, it
78	     performs well, but the issuing task is therefore required to stay on that
79	     CPU until it has finished, lest some other task displace its mappings.
80	
81	     kmap_atomic() may also be used by interrupt contexts, since it is does not
82	     sleep and the caller may not sleep until after kunmap_atomic() is called.
83	
84	     It may be assumed that k[un]map_atomic() won't fail.
85	
86	
87	=================
88	USING KMAP_ATOMIC
89	=================
90	
91	When and where to use kmap_atomic() is straightforward.  It is used when code
92	wants to access the contents of a page that might be allocated from high memory
93	(see __GFP_HIGHMEM), for example a page in the pagecache.  The API has two
94	functions, and they can be used in a manner similar to the following:
95	
96		/* Find the page of interest. */
97		struct page *page = find_get_page(mapping, offset);
98	
99		/* Gain access to the contents of that page. */
100		void *vaddr = kmap_atomic(page);
101	
102		/* Do something to the contents of that page. */
103		memset(vaddr, 0, PAGE_SIZE);
104	
105		/* Unmap that page. */
106		kunmap_atomic(vaddr);
107	
108	Note that the kunmap_atomic() call takes the result of the kmap_atomic() call
109	not the argument.
110	
111	If you need to map two pages because you want to copy from one page to
112	another you need to keep the kmap_atomic calls strictly nested, like:
113	
114		vaddr1 = kmap_atomic(page1);
115		vaddr2 = kmap_atomic(page2);
116	
117		memcpy(vaddr1, vaddr2, PAGE_SIZE);
118	
119		kunmap_atomic(vaddr2);
120		kunmap_atomic(vaddr1);
121	
122	
123	==========================
124	COST OF TEMPORARY MAPPINGS
125	==========================
126	
127	The cost of creating temporary mappings can be quite high.  The arch has to
128	manipulate the kernel's page tables, the data TLB and/or the MMU's registers.
129	
130	If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping
131	simply with a bit of arithmetic that will convert the page struct address into
132	a pointer to the page contents rather than juggling mappings about.  In such a
133	case, the unmap operation may be a null operation.
134	
135	If CONFIG_MMU is not set, then there can be no temporary mappings and no
136	highmem.  In such a case, the arithmetic approach will also be used.
137	
138	
139	========
140	i386 PAE
141	========
142	
143	The i386 arch, under some circumstances, will permit you to stick up to 64GiB
144	of RAM into your 32-bit machine.  This has a number of consequences:
145	
146	 (*) Linux needs a page-frame structure for each page in the system and the
147	     pageframes need to live in the permanent mapping, which means:
148	
149	 (*) you can have 896M/sizeof(struct page) page-frames at most; with struct
150	     page being 32-bytes that would end up being something in the order of 112G
151	     worth of pages; the kernel, however, needs to store more than just
152	     page-frames in that memory...
153	
154	 (*) PAE makes your page tables larger - which slows the system down as more
155	     data has to be accessed to traverse in TLB fills and the like.  One
156	     advantage is that PAE has more PTE bits and can provide advanced features
157	     like NX and PAT.
158	
159	The general recommendation is that you don't use more than 8GiB on a 32-bit
160	machine - although more might work for you and your workload, you're pretty
161	much on your own - don't expect kernel developers to really care much if things
162	come apart.
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