[Linux kernel memory management] Partition partner allocator ⑤ (zone watermark | zone watermark data structure zone_watermarks enumeration | zone watermark member in the memory zone zone)

1. Regional waterline

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The "preferred memory area " borrows physical memory from the "alternate memory area " in a specific case, which is related to the "area watermark" ;

Each "memory area" has $3$ water lines:

① High water mark: High Water Marker, the number of free pages in the memory area is greater than the high water mark , and the memory is sufficient ;

② Low watermark: Low Water Marker, the number of free pages in the memory area is less than the low watermark , and the memory is slightly insufficient ;

③ Minimum water mark: Min Water Marker, the number of free pages in the memory area is less than the minimum water mark , and the memory is seriously insufficient;

The memory below the minimum waterline is "emergency reserved memory" , which will only be allocated to specific processes when the memory is seriously insufficient , and these processes must promise to "allocate a small amount of memory and release more memory";

2. Zone watermark data structure zone_watermarks enumeration ( WMARK_MIN | WMARK_LOW | WMARK_HIGH | NR_WMARK )

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The data structure corresponding to the zone waterline is defined in the linux-4.12\include\linux\mmzone.h #255 location of the linux kernel source code , which is an enumeration;

enum zone_watermarks {
    
    
	WMARK_MIN,
	WMARK_LOW,
	WMARK_HIGH,
	NR_WMARK
};

Source code path: linux-4.12\include\linux\mmzone.h #255

3. The regional watermark members in the memory area zone

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The member in the "memory area" structurestruct zone is the area watermark used by the "page allocator" in the memory area;unsigned long watermark[NR_WMARK];

struct zone {
    
    
	/* Read-mostly fields */

	/* zone watermarks, access with *_wmark_pages(zone) macros */
	unsigned long watermark[NR_WMARK];
}

Source code path: linux-4.12\include\linux\mmzone.h #354