What is field map in free_area_t?

[Timur Tabi <ttabi@interactivesi.com>]

Before I ask my question, I'd like to thank everyone who's helped me so far.  I
really appreciate all this help, and it is making a big difference in my ability
to understand the Linux kernel.  I look forward to the day when I can answer
other people's questions, and once I'm done, I'm planning on writing a document
that explains what I've learned.

Could someone explain to me what the "map" field of struct free_area_t does? 
It's defined in mmzone.h:

typedef struct free_area_struct {
	struct list_head	free_list;
	unsigned int	*map;
} free_area_t;

It appears to be a pointer to some kind of field of bits, but that's all I
could figure out.  It's used in two places:

page_alloc.c, function __free_pages_ok:

	if (!test_and_change_bit(index, area->map))

and in function free_area_init_core:

	zone->free_area[i].map = 
	  (unsigned int *) alloc_bootmem_node(nid, bitmap_size);

It's also part of the MARK_USED macro:

#define MARK_USED(index, order, area) \
	change_bit((index) >> (1+(order)), (area)->map)

which is used in functions expand() and rmqueue().



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Timur Tabi - ttabi@interactivesi.com
Interactive Silicon - http://www.interactivesi.com

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