/*
 * 直接分配内存块失败，进入慢速路径
 * */
static inline struct page *
__alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
						struct alloc_context *ac)
{
	///是否支持直接回收，GFP_KERNEL,GFP_HIGHUSER_MOVABLE等
	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
        ///判断当前请求的页面分配是否为高成本的页面请求（即分配多个页面）
	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
        // 初始化一个指针，用来保存分配的页面地址
	struct page *page = NULL;
        // 用于存储分配的标志
	unsigned int alloc_flags;
        // 记录回收过程是否有进展
	unsigned long did_some_progress;
        // 用于指定页面整理（compaction）时的优先级
	enum compact_priority compact_priority;
        // 用于保存整理操作的结果
	enum compact_result compact_result;
        // 用于记录页面整理操作的重试次数
	int compaction_retries;
        // 用于记录没有进展的循环次数
	int no_progress_loops;
        // 用于处理 CPU 集合更新时的竞争条件
	unsigned int cpuset_mems_cookie;
        // 用于表示系统是否允许访问内存预留区域
	int reserve_flags;

	/*
	 * We also sanity check to catch abuse of atomic reserves being used by
	 * callers that are not in atomic context.
	 */
	 ///检查__GFP_ATOMIC,用在中断上下文，优先级较高，检查是否滥用，这两个标签是无法同时使用的，如果同时使用，则发出警告并去除GFP_ATOMIC
	if (WARN_ON_ONCE((gfp_mask & (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)) ==
				(__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
		gfp_mask &= ~__GFP_ATOMIC;

retry_cpuset:
	compaction_retries = 0;
	no_progress_loops = 0;
	compact_priority = DEF_COMPACT_PRIORITY;
	cpuset_mems_cookie = read_mems_allowed_begin();

	/*
	 * The fast path uses conservative alloc_flags to succeed only until
	 * kswapd needs to be woken up, and to avoid the cost of setting up
	 * alloc_flags precisely. So we do that now.
	 */
	 ///因为低水位分配失败，才走到这里，修改标志为最低水位线，后面重新尝试分配
	alloc_flags = gfp_to_alloc_flags(gfp_mask);

	/*
	 * We need to recalculate the starting point for the zonelist iterator
	 * because we might have used different nodemask in the fast path, or
	 * there was a cpuset modification and we are retrying - otherwise we
	 * could end up iterating over non-eligible zones endlessly.
	 */
	 ///重新选择优先zone
	ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
					ac->highest_zoneidx, ac->nodemask);
	if (!ac->preferred_zoneref->zone)
		goto nopage;

	///唤醒kswapd内核线程，回收页面
	if (alloc_flags & ALLOC_KSWAPD)
		wake_all_kswapds(order, gfp_mask, ac);

	/*
	 * The adjusted alloc_flags might result in immediate success, so try
	 * that first
	 */
	///kswapd回收后，再次尝试直接内存分配
	page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
	if (page)
		goto got_pg;

	/*
	 * For costly allocations, try direct compaction first, as it's likely
	 * that we have enough base pages and don't need to reclaim. For non-
	 * movable high-order allocations, do that as well, as compaction will
	 * try prevent permanent fragmentation by migrating from blocks of the
	 * same migratetype.
	 * Don't try this for allocations that are allowed to ignore
	 * watermarks, as the ALLOC_NO_WATERMARKS attempt didn't yet happen.
	 */
	if (can_direct_reclaim &&
			(costly_order ||
			   (order > 0 && ac->migratetype != MIGRATE_MOVABLE))
			&& !gfp_pfmemalloc_allowed(gfp_mask)) {
		///进行页面规整,异步模式,触发条件：
		///允许直接回收
		///高成本，大order页分配需求
		///不能访问系统预留内存
		page = __alloc_pages_direct_compact(gfp_mask, order,
						alloc_flags, ac,
						INIT_COMPACT_PRIORITY,
						&compact_result);
		if (page)
			goto got_pg;

		/*
		 * Checks for costly allocations with __GFP_NORETRY, which
		 * includes some THP page fault allocations
		 */
		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
			/*
			 * If allocating entire pageblock(s) and compaction
			 * failed because all zones are below low watermarks
			 * or is prohibited because it recently failed at this
			 * order, fail immediately unless the allocator has
			 * requested compaction and reclaim retry.
			 *
			 * Reclaim is
			 *  - potentially very expensive because zones are far
			 *    below their low watermarks or this is part of very
			 *    bursty high order allocations,
			 *  - not guaranteed to help because isolate_freepages()
			 *    may not iterate over freed pages as part of its
			 *    linear scan, and
			 *  - unlikely to make entire pageblocks free on its
			 *    own.
			 */
			if (compact_result == COMPACT_SKIPPED ||
			    compact_result == COMPACT_DEFERRED)
				goto nopage;

			/*
			 * Looks like reclaim/compaction is worth trying, but
			 * sync compaction could be very expensive, so keep
			 * using async compaction.
			 */
			compact_priority = INIT_COMPACT_PRIORITY;
		}
	}

retry:
	/* Ensure kswapd doesn't accidentally go to sleep as long as we loop */
	if (alloc_flags & ALLOC_KSWAPD)
		///唤醒kswapd内核线程
		wake_all_kswapds(order, gfp_mask, ac);

	///修改分配属性，可以分配系统预留内存
	reserve_flags = __gfp_pfmemalloc_flags(gfp_mask);
	if (reserve_flags)
		alloc_flags = gfp_to_alloc_flags_cma(gfp_mask, reserve_flags);

	/*
	 * Reset the nodemask and zonelist iterators if memory policies can be
	 * ignored. These allocations are high priority and system rather than
	 * user oriented.
	 */
	if (!(alloc_flags & ALLOC_CPUSET) || reserve_flags) {
		ac->nodemask = NULL;
		ac->preferred_zoneref = first_zones_zonelist(ac->zonelist,
					ac->highest_zoneidx, ac->nodemask);
	}

	///kswapd工作后，再尝试分配
	/* Attempt with potentially adjusted zonelist and alloc_flags */
	page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
	if (page)
		goto got_pg;

	/* Caller is not willing to reclaim, we can't balance anything */
	if (!can_direct_reclaim)
		goto nopage;

	/* Avoid recursion of direct reclaim */
	if (current->flags & PF_MEMALLOC)
		goto nopage;

	///触发直接内存回收
	/* Try direct reclaim and then allocating */
	page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac,
							&did_some_progress);
	if (page)
		goto got_pg;

	///再次尝试页面规整，然后重新分配内存
	/* Try direct compaction and then allocating */
	page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags, ac,
					compact_priority, &compact_result);
	if (page)
		goto got_pg;

	/* Do not loop if specifically requested */
	if (gfp_mask & __GFP_NORETRY)
		goto nopage;

	/*
	 * Do not retry costly high order allocations unless they are
	 * __GFP_RETRY_MAYFAIL
	 */
	if (costly_order && !(gfp_mask & __GFP_RETRY_MAYFAIL))
		goto nopage;

	///重试直接内存回收,会增加no_progress_loops计数
	if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
				 did_some_progress > 0, &no_progress_loops))
		goto retry;

	/*
	 * It doesn't make any sense to retry for the compaction if the order-0
	 * reclaim is not able to make any progress because the current
	 * implementation of the compaction depends on the sufficient amount
	 * of free memory (see __compaction_suitable)
	 */
	 ///是否重新内存规整
	if (did_some_progress > 0 &&
			should_compact_retry(ac, order, alloc_flags,
				compact_result, &compact_priority,
				&compaction_retries))
		goto retry;


	/* Deal with possible cpuset update races before we start OOM killing */
	if (check_retry_cpuset(cpuset_mems_cookie, ac))
		goto retry_cpuset;

	///回收，页面规整都失败，尝试启动oom
	/* Reclaim has failed us, start killing things */
	page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
	if (page)
		goto got_pg;

	/* Avoid allocations with no watermarks from looping endlessly */
	if (tsk_is_oom_victim(current) &&
	    (alloc_flags & ALLOC_OOM ||
	     (gfp_mask & __GFP_NOMEMALLOC)))
		goto nopage;

	/* Retry as long as the OOM killer is making progress */
	if (did_some_progress) {
		no_progress_loops = 0;
		goto retry;
	}

nopage:
	/* Deal with possible cpuset update races before we fail */
	if (check_retry_cpuset(cpuset_mems_cookie, ac))
		goto retry_cpuset;

	/*
	 * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
	 * we always retry
	 */
	if (gfp_mask & __GFP_NOFAIL) {
		/*
		 * All existing users of the __GFP_NOFAIL are blockable, so warn
		 * of any new users that actually require GFP_NOWAIT
		 */
		if (WARN_ON_ONCE(!can_direct_reclaim))
			goto fail;

		/*
		 * PF_MEMALLOC request from this context is rather bizarre
		 * because we cannot reclaim anything and only can loop waiting
		 * for somebody to do a work for us
		 */
		WARN_ON_ONCE(current->flags & PF_MEMALLOC);

		/*
		 * non failing costly orders are a hard requirement which we
		 * are not prepared for much so let's warn about these users
		 * so that we can identify them and convert them to something
		 * else.
		 */
		WARN_ON_ONCE(order > PAGE_ALLOC_COSTLY_ORDER);

		/*
		 * Help non-failing allocations by giving them access to memory
		 * reserves but do not use ALLOC_NO_WATERMARKS because this
		 * could deplete whole memory reserves which would just make
		 * the situation worse
		 */
		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_HARDER, ac);
		if (page)
			goto got_pg;

		cond_resched();
		goto retry;
	}
fail:
///所有分配尝试都失败，打印函数调用栈，系统信息
	warn_alloc(gfp_mask, ac->nodemask,
			"page allocation failure: order:%u", order);
got_pg:
	return page;
}

if (WARN_ON_ONCE((gfp_mask & (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)) ==
				(__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
		gfp_mask &= ~__GFP_ATOMIC;

if (can_direct_reclaim &&
			(costly_order ||
			   (order > 0 && ac->migratetype != MIGRATE_MOVABLE))
			&& !gfp_pfmemalloc_allowed(gfp_mask)) {
		page = __alloc_pages_direct_compact(gfp_mask, order,
						alloc_flags, ac,
						INIT_COMPACT_PRIORITY,
						&compact_result);

fail:
///所有分配尝试都失败，打印函数调用栈，系统信息
	warn_alloc(gfp_mask, ac->nodemask,
			"page allocation failure: order:%u", order);


void warn_alloc(gfp_t gfp_mask, nodemask_t *nodemask, const char *fmt, ...)
{
	struct va_format vaf;
	va_list args;
	static DEFINE_RATELIMIT_STATE(nopage_rs, 10*HZ, 1);
 
	if ((gfp_mask & __GFP_NOWARN) ||
	     !__ratelimit(&nopage_rs) ||
	     ((gfp_mask & __GFP_DMA) && !has_managed_dma()))
		return;
 
	va_start(args, fmt);
	vaf.fmt = fmt;
	vaf.va = &args;
	pr_warn("%s: %pV, mode:%#x(%pGg), nodemask=%*pbl",
			current->comm, &vaf, gfp_mask, &gfp_mask,
			nodemask_pr_args(nodemask));
	va_end(args);
 
	cpuset_print_current_mems_allowed();
	pr_cont("\n");
	dump_stack();
	warn_alloc_show_mem(gfp_mask, nodemask);
}