区zone的水位检查__zone_watermark

区zone的水位检查__zone_watermark_ok

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在Linux内核的内存管理中，整个内存以zone为单位进行维护，函数__zone_watermark_ok()

用于对zone的水位进行检查，其主要是处理min和free_pages两个关系。

函数根据不同条件缩减或者扩张两个对象。

函数实现如下

mm/page_alloc.c
函数返回真，如果水位检查ok，否则返回false。
需要注意对order-0和order非零检查的不同。
/*
* Return true if free base pages are above 'mark'. For high-order checks it
* will return true of the order-0 watermark is reached and there is at least
* one free page of a suitable size. Checking now avoids taking the zone lock
* to check in the allocation paths if no pages are free.
*/
bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
int classzone_idx, unsigned int alloc_flags,
long free_pages)
{
long min = mark;
int o;
const bool alloc_harder = (alloc_flags & ALLOC_HARDER);
 /* free_pages may go negative - that's OK */
free_pages -= (1 << order) - 1;
 if (alloc_flags & ALLOC_HIGH)
 min -= min / 2;
 /*
* If the caller does not have rights to ALLOC_HARDER then subtract
* the high-atomic reserves. This will over-estimate the size of the
* atomic reserve but it avoids a search.
*/
 if (likely(!alloc_harder))
free_pages -= z->nr_reserved_highatomic;
else
min -= min / 4;
#ifdef CONFIG_CMA
/* If allocation can't use CMA areas don't use free CMA pages */
if (!(alloc_flags & ALLOC_CMA))
 free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);
#endif
 /*
* Check watermarks for an order-0 allocation request. If these
* are not met, then a high-order request also cannot go ahead
* even if a suitable page happened to be free.
*/
 if (free_pages <= min + z->lowmem_reserve[classzone_idx])
return false;
 /* If this is an order-0 request then the watermark is fine */
 if (!order)
return true;
 /* For a high-order request, check at least one suitable page is free */
 for (o = order; o < MAX_ORDER; o++) {
struct free_area *area = &z->free_area[o];
int mt;
  if (!area->nr_free)
continue;
  if (alloc_harder)
return true;
  for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) {
if (!list_empty(&area->free_list[mt]))
return true;
}
#ifdef CONFIG_CMA
 if ((alloc_flags & ALLOC_CMA) &&
!list_empty(&area->free_list[MIGRATE_CMA])) {
return true;
}
#endif
 }
return false;
}