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概述 

《Linux内存回收入口_nginux的博客-CSDN博客》前文我们概略的描述了几种内存回收入口，我们知道几种回收入口最终都会调用进入shrink_node函数，本文将以Linux 5.9源码来描述shrink_node函数的源码实现。

函数调用流程图

 scan_control数据结构

struct scan_control {/* How many pages shrink_list() should reclaim */unsigned long nr_to_reclaim;/** Nodemask of nodes allowed by the caller. If NULL, all nodes* are scanned.*/nodemask_t	*nodemask;/** The memory cgroup that hit its limit and as a result is the* primary target of this reclaim invocation.*/struct mem_cgroup *target_mem_cgroup;/** Scan pressure balancing between anon and file LRUs*/unsigned long	anon_cost;unsigned long	file_cost;/* Can active pages be deactivated as part of reclaim? *///是否能从active lru列表进行deactivate的reclaim
#define DEACTIVATE_ANON 1
#define DEACTIVATE_FILE 2unsigned int may_deactivate:2;//如果是1：代表强制进行deactivate，即同时deactivate file和anon//如果是0，按需进行deactivate file或者anon，具体条件见下面shrink_node源码分析unsigned int force_deactivate:1;unsigned int skipped_deactivate:1;/* Writepage batching in laptop mode; RECLAIM_WRITE */unsigned int may_writepage:1;/* Can mapped pages be reclaimed? */unsigned int may_unmap:1;/* Can pages be swapped as part of reclaim? */unsigned int may_swap:1;/** Cgroups are not reclaimed below their configured memory.low,* unless we threaten to OOM. If any cgroups are skipped due to* memory.low and nothing was reclaimed, go back for memory.low.*/unsigned int memcg_low_reclaim:1;unsigned int memcg_low_skipped:1;unsigned int hibernation_mode:1;/* One of the zones is ready for compaction */unsigned int compaction_ready:1;/* There is easily reclaimable cold cache in the current node *///设置为1代表只回收file page cache，不回收aone pageunsigned int cache_trim_mode:1;/* The file pages on the current node are dangerously low *///设置1代表只回收aone page，不回收file pageunsigned int file_is_tiny:1;/* Allocation order */s8 order;/* Scan (total_size >> priority) pages at once */s8 priority;/* The highest zone to isolate pages for reclaim from */s8 reclaim_idx;/* This context's GFP mask */gfp_t gfp_mask;/* Incremented by the number of inactive pages that were scanned */unsigned long nr_scanned;/* Number of pages freed so far during a call to shrink_zones() */unsigned long nr_reclaimed;struct {unsigned int dirty;unsigned int unqueued_dirty;unsigned int congested;unsigned int writeback;unsigned int immediate;unsigned int file_taken;unsigned int taken;} nr;/* for recording the reclaimed slab by now */struct reclaim_state reclaim_state;
};

shrink_node函数

static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
{struct reclaim_state *reclaim_state = current->reclaim_state;unsigned long nr_reclaimed, nr_scanned;struct lruvec *target_lruvec;bool reclaimable = false;unsigned long file;target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);again:memset(&sc->nr, 0, sizeof(sc->nr));nr_reclaimed = sc->nr_reclaimed;nr_scanned = sc->nr_scanned;/** Determine the scan balance between anon and file LRUs.*/spin_lock_irq(&pgdat->lru_lock);sc->anon_cost = target_lruvec->anon_cost;sc->file_cost = target_lruvec->file_cost;spin_unlock_irq(&pgdat->lru_lock);/** Target desirable inactive:active list ratios for the anon* and file LRU lists.*/if (!sc->force_deactivate) {unsigned long refaults;refaults = lruvec_page_state(target_lruvec,WORKINGSET_ACTIVATE_ANON);//anon的refaults值比上次回收发生了变化，或者inactive anon很少，设置//DEACTIVATE_ANON表示需要deactivate anonif (refaults != target_lruvec->refaults[0] ||inactive_is_low(target_lruvec, LRU_INACTIVE_ANON))sc->may_deactivate |= DEACTIVATE_ANON;elsesc->may_deactivate &= ~DEACTIVATE_ANON;/** When refaults are being observed, it means a new* workingset is being established. Deactivate to get* rid of any stale active pages quickly.*/refaults = lruvec_page_state(target_lruvec,WORKINGSET_ACTIVATE_FILE);if (refaults != target_lruvec->refaults[1] ||inactive_is_low(target_lruvec, LRU_INACTIVE_FILE))sc->may_deactivate |= DEACTIVATE_FILE;elsesc->may_deactivate &= ~DEACTIVATE_FILE;} elsesc->may_deactivate = DEACTIVATE_ANON | DEACTIVATE_FILE;/** If we have plenty of inactive file pages that aren't* thrashing, try to reclaim those first before touching* anonymous pages.*///file是inactive file的数量file = lruvec_page_state(target_lruvec, NR_INACTIVE_FILE);if (file >> sc->priority && !(sc->may_deactivate & DEACTIVATE_FILE))//只回收file page，影响get_scan_countsc->cache_trim_mode = 1;elsesc->cache_trim_mode = 0;/** Prevent the reclaimer from falling into the cache trap: as* cache pages start out inactive, every cache fault will tip* the scan balance towards the file LRU.  And as the file LRU* shrinks, so does the window for rotation from references.* This means we have a runaway feedback loop where a tiny* thrashing file LRU becomes infinitely more attractive than* anon pages.  Try to detect this based on file LRU size.*/if (!cgroup_reclaim(sc)) {unsigned long total_high_wmark = 0;unsigned long free, anon;int z;free = sum_zone_node_page_state(pgdat->node_id, NR_FREE_PAGES);file = node_page_state(pgdat, NR_ACTIVE_FILE) +node_page_state(pgdat, NR_INACTIVE_FILE);for (z = 0; z < MAX_NR_ZONES; z++) {struct zone *zone = &pgdat->node_zones[z];if (!managed_zone(zone))continue;total_high_wmark += high_wmark_pages(zone);}/** Consider anon: if that's low too, this isn't a* runaway file reclaim problem, but rather just* extreme pressure. Reclaim as per usual then.*/anon = node_page_state(pgdat, NR_INACTIVE_ANON);//设置1代表只回收aone page，不回收file pagesc->file_is_tiny =file + free <= total_high_wmark &&!(sc->may_deactivate & DEACTIVATE_ANON) &&anon >> sc->priority;}//回收的核心函数，后面文章专门分析shrink_node_memcgs(pgdat, sc);if (reclaim_state) {sc->nr_reclaimed += reclaim_state->reclaimed_slab;reclaim_state->reclaimed_slab = 0;}/* Record the subtree's reclaim efficiency */vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,sc->nr_scanned - nr_scanned,sc->nr_reclaimed - nr_reclaimed);//这一轮回收到了页面if (sc->nr_reclaimed - nr_reclaimed)reclaimable = true;//只允许kswapd线程设置这些flag，因为只有kswapd能clear这些flag，避免混乱//比如memcg reclaim也能设置，没法保证kswapd肯定会被wakeup去clear这些标志if (current_is_kswapd()) {/** If reclaim is isolating dirty pages under writeback,* it implies that the long-lived page allocation rate* is exceeding the page laundering rate. Either the* global limits are not being effective at throttling* processes due to the page distribution throughout* zones or there is heavy usage of a slow backing* device. The only option is to throttle from reclaim* context which is not ideal as there is no guarantee* the dirtying process is throttled in the same way* balance_dirty_pages() manages.** Once a node is flagged PGDAT_WRITEBACK, kswapd will* count the number of pages under pages flagged for* immediate reclaim and stall if any are encountered* in the nr_immediate check below.*///设置PGDAT_DIRTY代表reclaim发现很多页面正在回写if (sc->nr.writeback && sc->nr.writeback == sc->nr.taken)set_bit(PGDAT_WRITEBACK, &pgdat->flags);/* Allow kswapd to start writing pages during reclaim.*/设置PGDAT_DIRTY代表reclaim发现很多脏页if (sc->nr.unqueued_dirty == sc->nr.file_taken)set_bit(PGDAT_DIRTY, &pgdat->flags);/** If kswapd scans pages marked for immediate* reclaim and under writeback (nr_immediate), it* implies that pages are cycling through the LRU* faster than they are written so also forcibly stall.*/if (sc->nr.immediate)congestion_wait(BLK_RW_ASYNC, HZ/10);}/** Tag a node/memcg as congested if all the dirty pages* scanned were backed by a congested BDI and* wait_iff_congested will stall.** Legacy memcg will stall in page writeback so avoid forcibly* stalling in wait_iff_congested().*///只允许kswapd线程设置LRUVEC_CONGESTED，因为只有kswapd能clear LRUVEC_CONGESTED，//比如memcg reclaim也能设置，没法保证kswap能唤醒去clear LRUVEC_CONGESTED，导致//direct reclaim阻塞在wait_iff_congestedif ((current_is_kswapd() ||(cgroup_reclaim(sc) && writeback_throttling_sane(sc))) &&sc->nr.dirty && sc->nr.dirty == sc->nr.congested)set_bit(LRUVEC_CONGESTED, &target_lruvec->flags);/** Stall direct reclaim for IO completions if underlying BDIs* and node is congested. Allow kswapd to continue until it* starts encountering unqueued dirty pages or cycling through* the LRU too quickly.*///如果是非kswapd线程，且判定当前回收设置过拥塞flag，就要等待，所以direct reclaim//会被阻塞if (!current_is_kswapd() && current_may_throttle() &&!sc->hibernation_mode &&test_bit(LRUVEC_CONGESTED, &target_lruvec->flags))wait_iff_congested(BLK_RW_ASYNC, HZ/10);//如果需要继续回收，就goto again继续if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,sc))goto again;/** Kswapd gives up on balancing particular nodes after too* many failures to reclaim anything from them and goes to* sleep. On reclaim progress, reset the failure counter. A* successful direct reclaim run will revive a dormant kswapd.*/if (reclaimable)pgdat->kswapd_failures = 0;
}

should_continue_reclaim 

/** Reclaim/compaction is used for high-order allocation requests. It reclaims* order-0 pages before compacting the zone. should_continue_reclaim() returns* true if more pages should be reclaimed such that when the page allocator* calls try_to_compact_pages() that it will have enough free pages to succeed.* It will give up earlier than that if there is difficulty reclaiming pages.*/
static inline bool should_continue_reclaim(struct pglist_data *pgdat,unsigned long nr_reclaimed,struct scan_control *sc)
{unsigned long pages_for_compaction;unsigned long inactive_lru_pages;int z;/* If not in reclaim/compaction mode, stop */if (!in_reclaim_compaction(sc))return false;/** Stop if we failed to reclaim any pages from the last SWAP_CLUSTER_MAX* number of pages that were scanned. This will return to the caller* with the risk reclaim/compaction and the resulting allocation attempt* fails. In the past we have tried harder for __GFP_RETRY_MAYFAIL* allocations through requiring that the full LRU list has been scanned* first, by assuming that zero delta of sc->nr_scanned means full LRU* scan, but that approximation was wrong, and there were corner cases* where always a non-zero amount of pages were scanned.*/if (!nr_reclaimed)return false;//compaction_suitable会检查水位是否已满足条件（要根据orderPAGE_ALLOC_COSTLY_ORDER//使用不同的watermark,如果不满足就不会返回success/continue/* If compaction would go ahead or the allocation would succeed, stop */for (z = 0; z <= sc->reclaim_idx; z++) {struct zone *zone = &pgdat->node_zones[z];if (!managed_zone(zone))continue;//满足了水位return false，代表不要继续shrink_node了switch (compaction_suitable(zone, sc->order, 0, sc->reclaim_idx)) {case COMPACT_SUCCESS:case COMPACT_CONTINUE:return false;default:/* check next zone */;}}/** If we have not reclaimed enough pages for compaction and the* inactive lists are large enough, continue reclaiming*///上面水位检查不通过，且也没有reclaim足够的page来做compaction，那就继续reclaim吧pages_for_compaction = compact_gap(sc->order);inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);if (get_nr_swap_pages() > 0)inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);return inactive_lru_pages > pages_for_compaction;
}

 compaction_suitable会判定当前水位是否满足order申请，如果满足了那么COMPACT_SUCCESS，说明也不需要继续compact了；如果不满了说明还没有回到足够order申请的内存，逻辑会继续往下走到inactive_lru_pages > pages_for_compaction逻辑判定，如果inactive lru中有大于2被申请order的页面，那就继续扫描回收

参考文章：

[PATCH v2 4/4] mm/vmscan: Don't mess with pgdat->flags in memcg reclaim. - Andrey Ryabinin

Linux 内存管理_workingset内存_jianchwa的博客-CSDN博客