// file: include/linux/mm_types.h
...
struct page {
unsigned long flags; /* Atomic flags, some possibly
* updated asynchronously */
/*
* Five words (20/40 bytes) are available in this union.
* WARNING: bit 0 of the first word is used for PageTail(). That
* means the other users of this union MUST NOT use the bit to
* avoid collision and false-positive PageTail().
*/
union {
struct { /* Page cache and anonymous pages */
/**
* @lru: Pageout list, eg. active_list protected by
* pgdat->lru_lock. Sometimes used as a generic list
* by the page owner.
*/
struct list_head lru;
/* See page-flags.h for PAGE_MAPPING_FLAGS */
struct address_space *mapping;
pgoff_t index; /* Our offset within mapping. */
/**
* @private: Mapping-private opaque data.
* Usually used for buffer_heads if PagePrivate.
* Used for swp_entry_t if PageSwapCache.
* Indicates order in the buddy system if PageBuddy.
*/
unsigned long private;
};
....
} _struct_page_alignment;
占用空间大小
// file: mm/page_alloc.c
static unsigned long __init calc_memmap_size(unsigned long spanned_pages, unsigned long present_pages)
{
unsigned long pages = spanned_pages;
/*
* Provide a more accurate estimation if there are holes within
* the zone and SPARSEMEM is in use. If there are holes within the
* zone, each populated memory region may cost us one or two extra
* memmap pages due to alignment because memmap pages for each
* populated regions may not be naturally aligned on page boundary.
* So the (present_pages >> 4) heuristic is a tradeoff for that.
*/
if (spanned_pages > present_pages + (present_pages >> 4) &&
IS_ENABLED(CONFIG_SPARSEMEM))
pages = present_pages;
return PAGE_ALIGN(pages * sizeof(struct page)) >> PAGE_SHIFT;
}
static void __init free_area_init_core(struct pglist_data *pgdat)
{
enum zone_type j;
int nid = pgdat->node_id;
pgdat_init_internals(pgdat);
pgdat->per_cpu_nodestats = &boot_nodestats;
for (j = 0; j < MAX_NR_ZONES; j++) {
struct zone *zone = pgdat->node_zones + j;
unsigned long size, freesize, memmap_pages;
unsigned long zone_start_pfn = zone->zone_start_pfn;
size = zone->spanned_pages;
freesize = zone->present_pages;
/*
* Adjust freesize so that it accounts for how much memory
* is used by this zone for memmap. This affects the watermark
* and per-cpu initialisations
*/
memmap_pages = calc_memmap_size(size, freesize);
if (!is_highmem_idx(j)) {
if (freesize >= memmap_pages) {
freesize -= memmap_pages;
if (memmap_pages)
printk(KERN_DEBUG
" %s zone: %lu pages used for memmap\n",
zone_names[j], memmap_pages);
} else
pr_warn(" %s zone: %lu pages exceeds freesize %lu\n",
zone_names[j], memmap_pages, freesize);
}
....
}
}
