缓存为什么会有冷热？
究其原因，是因为对于内存的访问，可能是CPU发起的，也可以是DMA设备发起的。
如果是CPU发起的，在CPU的硬件缓存中，就会保存相应的页内容。如果这个页本来没有存在于硬件缓存中，那么它的到来，势必会将原本为其他的页缓存的内容挤出硬件缓存。
但是，如果对于内存的访问是由DMA设备发起的，那么该页不会被CPU访问，就不需要在CPU的硬件缓存中进行缓存，也不会对已经缓存在硬件缓存中的页内容造成伤害。
在Linux操作系统中，每个内存区域(Zone)都分配了hot cache和cold cache，hot cache用来缓存那些很可能被CPU的硬件缓存收纳了的页。
hot/cold cache只处理单页分配的情况。
1: /*
2: * Really, prep_compound_page() should be called from __rmqueue_bulk(). But
3: * we cheat by calling it from here, in the order > 0 path. Saves a branch
4: * or two.
5: */
6: static inline
7: struct page *buffered_rmqueue(struct zone *preferred_zone,
8: struct zone *zone, int order, gfp_t gfp_flags,
9: int migratetype)
10: { 11: unsigned long flags;
12: struct page *page;
13: int cold = !!(gfp_flags & __GFP_COLD);
14:
15: again:
16: if (likely(order == 0)) { 17: struct per_cpu_pages *pcp;
18: struct list_head *list;
19:
20: local_irq_save(flags);
21: pcp = &this_cpu_ptr(zone->pageset)->pcp;
22: list = &pcp->lists[migratetype];
23: if (list_empty(list)) { 24: pcp->count += rmqueue_bulk(zone, 0,
25: pcp->batch, list,
26: migratetype, cold);
27: if (unlikely(list_empty(list)))
28: goto failed;
29: }
30:
31: if (cold)
32: page = list_entry(list->prev, struct page, lru);
33: else
34: page = list_entry(list->next, struct page, lru);
35:
36: list_del(&page->lru);
37: pcp->count--;
38: } else { 39: if (unlikely(gfp_flags & __GFP_NOFAIL)) { 40: /*
41: * __GFP_NOFAIL is not to be used in new code.
42: *
43: * All __GFP_NOFAIL callers should be fixed so that they
44: * properly detect and handle allocation failures.
45: *
46: * We most definitely don't want callers attempting to
47: * allocate greater than order-1 page units with
48: * __GFP_NOFAIL.
49: */
50: WARN_ON_ONCE(order > 1);
51: }
52: spin_lock_irqsave(&zone->lock, flags);
53: page = __rmqueue(zone, order, migratetype);
54: spin_unlock(&zone->lock);
55: if (!page)
56: goto failed;
57: __mod_zone_page_state(zone, NR_FREE_PAGES, -(1 << order));
58: }
59:
60: __count_zone_vm_events(PGALLOC, zone, 1 << order);
61: zone_statistics(preferred_zone, zone, gfp_flags);
62: local_irq_restore(flags);
63:
64: VM_BUG_ON(bad_range(zone, page));
65: if (prep_new_page(page, order, gfp_flags))
66: goto again;
67: return page;
68:
69: failed:
70: local_irq_restore(flags);
71: return NULL;
72: }
buffered_rmqueue用于从冷热分配器中分配单页的缓存页。
如果gfp_flags中指定的__GFP_COLD，则从冷缓存中分配一页，否则，从热缓存中分配。