Re: sparsemem support for mips with highmem

[C Michael Sundius <Michael.sundius@sciatl.com>]

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Dave Hansen wrote:
> On Fri, 2008-08-15 at 10:16 -0700, C Michael Sundius wrote:
>   
>> Ah, your right. thanks.  "but it's not necessar*il*y a good idea".
>> That 
>> is to say, we don't put
>> memory above 2 GiB. No need to make the mem_section[] array bigger
>> than 
>> need be.
>>
>> This gives further credence for it to be a configurable in Kconfig as
>> well.
>>     
>
> I definitely don't want it to be something that users see.  It is never
> enough overhead to really care.  On a 16TB system with 16MB sections,
> the mem_section[] array is still only 16MB!!
>
> So, I'd say to just make it as big as the arch needs in the worst case
> (smallest SECTION_SIZE_BITS and largest MAX_PHYSMEM_BITS) and leave it.
> We might even want to merge the 32 and 64-bit versions.
>
> For your 32-bit version, we now use:
> 8 bytes (2 32-bit words) for each mem_section[]
> 2GB/128MB sections = 16
> So, that's only 512 bytes.
>
> For the 64-bit version, we now use:
> 16 bytes (2 64-bit words) for each mem_section[]
> 32GB/256MB sections = 128
> So, that's only 2048 bytes.
>
> If we were to merge the 32 and 64-bit versions to:
> #define SECTION_SIZE_BITS       27
> #define MAX_PHYSMEM_BITS        35
>
> Your 32-bit version would go to 2048 bytes, and the 64-bit version would
> go to 4096 bytes.  The 32-bit version would we able to address more
> memory, and the 64-bit version would be able to handle smaller memory
> holes more efficiently. 
>
> -- Dave
>
>   
Ah, compromise :] that's why you get paid the big bux dave. thanks.



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diff --git a/Documentation/sparsemem.txt b/Documentation/sparsemem.txt
new file mode 100644
index 0000000..89656e3
--- /dev/null
+++ b/Documentation/sparsemem.txt
@@ -0,0 +1,96 @@
+Sparsemem divides up physical memory in your system into N section of M
+bytes. Page descriptors are created for only those sections that
+actually exist (as far as the sparsemem code is concerned). This allows
+for holes in the physical memory without having to waste space by
+creating page discriptors for those pages that do not exist.
+When page_to_pfn() or pfn_to_page() are called there is a bit of overhead to
+look up the proper memory section to get to the descriptors, but this
+is small compared to the memory you are likely to save. So, it's not the
+default, but should be used if you have big holes in physical memory.
+
+Note that discontiguous memory is more closely related to NUMA machines
+and if you are a single CPU system use sparsemem and not discontig. 
+It's much simpler. 
+
+1) CALL MEMORY_PRESENT()
+Once the bootmem allocator is up and running, you should call the
+sparsemem function "memory_present(node, pfn_start, pfn_end)" for each
+block of memory that exists on your system.
+
+2) DETERMINE AND SET THE SIZE OF SECTIONS AND PHYSMEM
+The size of N and M above depend upon your architecture
+and your platform and are specified in the file:
+
+      include/asm-<your_arch>/sparsemem.h
+
+and you should create the following lines similar to below: 
+
+	#define SECTION_SIZE_BITS       27	/* 128 MiB */
+	#define MAX_PHYSMEM_BITS        31	/* 2 GiB   */
+
+if they don't already exist, where: 
+
+ * SECTION_SIZE_BITS            2^M: how big each section will be
+ * MAX_PHYSMEM_BITS             2^N: how much memory we can have in that
+                                     space
+
+Section size should be equal or less than the smallest block of
+memory in your system. Max physmem should be greater than or 
+equal to the highest physical memory address of memory in your
+system.
+
+3) INITIALIZE SPARSE MEMORY
+You should make sure that you initialize the sparse memory code by calling 
+
+	bootmem_init();
+  +	sparse_init();
+	paging_init();
+
+just before you call paging_init() and after the bootmem_allocator is
+turned on in your setup_arch() code.  
+
+4) ENABLE SPARSEMEM IN KCONFIG
+Add a line like this:
+
+	select ARCH_SPARSEMEM_ENABLE
+
+into the config for your platform in arch/<your_arch>/Kconfig. This will
+ensure that turning on sparsemem is enabled for your platform. 
+
+5) CONFIG
+Run make menuconfig or make gconfig, as you like, and turn on the sparsemem
+memory model under the "Kernel Type" --> "Memory Model" and then build your
+kernel.
+
+
+6) Gotchas
+
+One trick that I encountered when I was turning this on for MIPS was that there
+was some code in mem_init() that set the "reserved" flag for pages that were not
+valid RAM. This caused my kernel to crash when I enabled sparsemem since those
+pages (and page descriptors) didn't actually exist. I changed my code by adding
+lines like below:
+
+
+	for (tmp = highstart_pfn; tmp < highend_pfn; tmp++) {
+		struct page *page = pfn_to_page(tmp);
+
+   +		if (!pfn_valid(tmp))
+   +			continue;
+   +
+		if (!page_is_ram(tmp)) {
+			SetPageReserved(page);
+			continue;
+		}
+		ClearPageReserved(page);
+		init_page_count(page);
+		__free_page(page);
+		physmem_record(PFN_PHYS(tmp), PAGE_SIZE, physmem_highmem);
+		totalhigh_pages++;
+	}
+
+
+Once I got that straight, it worked!!!! I saved 10MiB of memory.  
+
+
+
diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index c6a063b..5b1af87 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -408,7 +408,6 @@ static void __init bootmem_init(void)
 
 		/* Register lowmem ranges */
 		free_bootmem(PFN_PHYS(start), size << PAGE_SHIFT);
-		memory_present(0, start, end);
 	}
 
 	/*
@@ -420,6 +419,23 @@ static void __init bootmem_init(void)
 	 * Reserve initrd memory if needed.
 	 */
 	finalize_initrd();
+
+	/* call memory present for all the ram */
+	for (i = 0; i < boot_mem_map.nr_map; i++) {
+		unsigned long start, end;
+
+		/*
+		 * memory present only usable memory.
+		 */
+		if (boot_mem_map.map[i].type != BOOT_MEM_RAM)
+			continue;
+
+		start = PFN_UP(boot_mem_map.map[i].addr);
+		end   = PFN_DOWN(boot_mem_map.map[i].addr
+				    + boot_mem_map.map[i].size);
+
+		memory_present(0, start, end);
+	}
 }
 
 #endif	/* CONFIG_SGI_IP27 */
diff --git a/arch/mips/mm/init.c b/arch/mips/mm/init.c
index 137c14b..31496a1 100644
--- a/arch/mips/mm/init.c
+++ b/arch/mips/mm/init.c
@@ -414,6 +414,9 @@ void __init mem_init(void)
 	for (tmp = highstart_pfn; tmp < highend_pfn; tmp++) {
 		struct page *page = pfn_to_page(tmp);
 
+		if (!pfn_valid(tmp))
+			continue;
+
 		if (!page_is_ram(tmp)) {
 			SetPageReserved(page);
 			continue;
diff --git a/include/asm-mips/sparsemem.h b/include/asm-mips/sparsemem.h
index 795ac6c..64376db 100644
--- a/include/asm-mips/sparsemem.h
+++ b/include/asm-mips/sparsemem.h
@@ -6,7 +6,7 @@
  * SECTION_SIZE_BITS		2^N: how big each section will be
  * MAX_PHYSMEM_BITS		2^N: how much memory we can have in that space
  */
-#define SECTION_SIZE_BITS       28
+#define SECTION_SIZE_BITS       27	/* 128 MiB */
 #define MAX_PHYSMEM_BITS        35
 
 #endif /* CONFIG_SPARSEMEM */