可以看到该函数根据分配标识__GFP_NOFAIL不断地调用get_page_from_freelist()循环尝试去获得内存。

接着回到__alloc_pages_slowpath()中，其从__alloc_pages_high_priority()退出后继而判断是否设置了__GFP_WAIT标识，如果设置则表示内存分配运行休眠，否则直接以分配内存失败而退出。接着将会调用__alloc_pages_direct_compact()和__alloc_pages_direct_reclaim()尝试回收内存并尝试分配。基于上面的多种尝试内存分配仍然失败的情况，将会调用__alloc_pages_may_oom()触发OOM killer机制。OOM killer将进程kill后会重新再次尝试内存分配，最后则是分配失败或分配成功的收尾处理。

__alloc_pages_slowpath()暂且分析至此，回到本文重点函数__alloc_pages_may_oom()中进一步进行分析。

【file:/ mm/page_alloc.h】
static inline struct page *
__alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
    struct zonelist *zonelist, enum zone_type high_zoneidx,
    nodemask_t *nodemask, struct zone *preferred_zone,
    int migratetype)
{
    struct page *page;
 
    /* Acquire the OOM killer lock for the zones in zonelist */
    if (!try_set_zonelist_oom(zonelist, gfp_mask)) {
        schedule_timeout_uninterruptible(1);
        return NULL;
    }
 
    /*
     * Go through the zonelist yet one more time, keep very high watermark
     * here, this is only to catch a parallel oom killing, we must fail if
     * we're still under heavy pressure.
     */
    page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask,
        order, zonelist, high_zoneidx,
        ALLOC_WMARK_HIGH|ALLOC_CPUSET,
        preferred_zone, migratetype);
    if (page)
        goto out;
 
    if (!(gfp_mask & __GFP_NOFAIL)) {
        /* The OOM killer will not help higher order allocs */
        if (order > PAGE_ALLOC_COSTLY_ORDER)
            goto out;
        /* The OOM killer does not needlessly kill tasks for lowmem */
        if (high_zoneidx < ZONE_NORMAL)
            goto out;
        /*
         * GFP_THISNODE contains __GFP_NORETRY and we never hit this.
         * Sanity check for bare calls of __GFP_THISNODE, not real OOM.
         * The caller should handle page allocation failure by itself if
         * it specifies __GFP_THISNODE.
         * Note: Hugepage uses it but will hit PAGE_ALLOC_COSTLY_ORDER.
         */
        if (gfp_mask & __GFP_THISNODE)
            goto out;
    }
    /* Exhausted what can be done so it's blamo time */
    out_of_memory(zonelist, gfp_mask, order, nodemask, false);
 
out:
    clear_zonelist_oom(zonelist, gfp_mask);
    return page;
}

该函数首先通过try_set_zonelist_oom()判断OOM killer是否已经在其他核进行killing操作，如果没有的情况下将会在try_set_zonelist_oom()内部进行锁操作，确保只有一个核执行killing的操作。继而调用get_page_from_freelist()在高watermark的情况下尝试再次获取内存，不过这里注定会失败。接着就是调用到了关键函数out_of_memory()。最后函数退出时将会调用clear_zonelist_oom()清除掉try_set_zonelist_oom()里面的锁操作。

着重分析一下out_of_memory()：