内存回收
内核主要有4种LRU链表：
enum lru_list {
LRU_INACTIVE_ANON = LRU_BASE,
LRU_ACTIVE_ANON = LRU_BASE + LRU_ACTIVE,
LRU_INACTIVE_FILE = LRU_BASE + LRU_FILE,
LRU_ACTIVE_FILE = LRU_BASE + LRU_FILE + LRU_ACTIVE,
LRU_UNEVICTABLE,
NR_LRU_LISTS
};
inactive_anon - # anonymous and swap cache memory on inactive LRU list.
active_anon - #anonymous and swap cache memory on active LRU list.
inactive_file - # file-backed memory on inactive LRU list.
active_file - # file-backed memory on active LRU list.
内核回收内存时，会在get_scan_out中计算每个链表中回收的page数量：
/*
* Determine how aggressively the anon and file LRU lists should be
* scanned. The relative value of each set of LRU lists is determined
* by looking at the fraction of the pages scanned we did rotate back
* onto the active list instead of evict.
*
* nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
* nr[2] = file inactive pages to scan; nr[3] = file active pages to scan
*/
static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
unsigned long *nr)
** （1）如果没有swap space，就不会回收anonymous page： **
/* If we have no swap space, do not bother scanning anon pages. */
if (!sc->may_swap || (get_nr_swap_pages() <= 0)) {
scan_balance = SCAN_FILE;
goto out;
}
** （2）如果是group回收，且memory.swappiness==0，则回收file cache： **
/*
* Global reclaim will swap to prevent OOM even with no
* swappiness, but memcg users want to use this knob to
* disable swapping for individual groups completely when
* using the memory controller's swap limit feature would be
* too expensive.
*/
if (!global_reclaim(sc) && !vmscan_swappiness(sc)) {
scan_balance = SCAN_FILE; ///mem cgroup swappiness == 0
goto out;
}
** （3）系统（group）内存已经不足，且swappiness!=0，则anon page和file cache都可以回收： **
/*
* Do not apply any pressure balancing cleverness when the
* system is close to OOM, scan both anon and file equally
* (unless the swappiness setting disagrees with swapping).
*/
if (!sc->priority && vmscan_swappiness(sc)) {
scan_balance = SCAN_EQUAL;
goto out;
}
** （4）如果空闲页(free)+file cache小于high water mark，则回收anon page（注：仅对全局回收）: **
anon = get_lru_size(lruvec, LRU_ACTIVE_ANON) +
get_lru_size(lruvec, LRU_INACTIVE_ANON);
file = get_lru_size(lruvec, LRU_ACTIVE_FILE) +
get_lru_size(lruvec, LRU_INACTIVE_FILE);
/*
* If it's foreseeable that reclaiming the file cache won't be
* enough to get the zone back into a desirable shape, we have
* to swap. Better start now and leave the - probably heavily
* thrashing - remaining file pages alone.
*/
if (global_reclaim(sc)) {
free = zone_page_state(zone, NR_FREE_PAGES);
if (unlikely(file + free <= high_wmark_pages(zone))) {
scan_balance = SCAN_ANON;
goto out;
}///file cache isn't enough, add ANON
}
** （5）inactive file cache足够，则回收file cache： **
/*
* There is enough inactive page cache, do not reclaim
* anything from the anonymous working set right now.
*/
if (!inactive_file_is_low(lruvec)) {
scan_balance = SCAN_FILE;
goto out;
}
** （6）其它情况，则根据swappiness计算回收anon page和file cache page的数量： **
scan_balance = SCAN_FRACT;
/*
* With swappiness at 100, anonymous and file have the same priority.
* This scanning priority is essentially the inverse of IO cost.
*/
anon_prio = vmscan_swappiness(sc);
file_prio = 200 - anon_prio;
