/*
 * Check that a process has enough memory to allocate a new virtual
 * mapping. 0 means there is enough memory for the allocation to
 * succeed and -ENOMEM implies there is not.
 *
 * We currently support three overcommit policies, which are set via the
 * vm.overcommit_memory sysctl.  See Documentation/vm/overcommit-accounting
 *
 * Strict overcommit modes added 2002 Feb 26 by Alan Cox.
 * Additional code 2002 Jul 20 by Robert Love.
 *
 * cap_sys_admin is 1 if the process has admin privileges, 0 otherwise.
 *
 * Note this is a helper function intended to be used by LSMs which
 * wish to use this logic.
 */
int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
{
        unsigned long free, allowed;

        vm_acct_memory(pages);

        /*
         * Sometimes we want to use more memory than we have
         */
        if (sysctl_overcommit_memory == OVERCOMMIT_ALWAYS)
                return 0;

        if (sysctl_overcommit_memory == OVERCOMMIT_GUESS) { //Heuristic overcommit算法开始
                unsigned long n;

                free = global_page_state(NR_FILE_PAGES); //pagecache汇总的页面数量
                free += get_nr_swap_pages(); //free swap的页面数

                /*
                 * Any slabs which are created with the
                 * SLAB_RECLAIM_ACCOUNT flag claim to have contents
                 * which are reclaimable, under pressure.  The dentry
                 * cache and most inode caches should fall into this
                 */
                free += global_page_state(NR_SLAB_RECLAIMABLE); //SLAB可回收的页面数

                /*
                 * Reserve some for root
                 */
                if (!cap_sys_admin)
                        free -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10); //给root用户保留的页面数

                if (free > pages)
                        return 0;

                /*
                 * nr_free_pages() is very expensive on large systems,
                 * only call if we're about to fail.
                 */
                n = nr_free_pages(); //当前free memory页面数

                /*
                 * Leave reserved pages. The pages are not for anonymous pages.
                 */
                if (n <= totalreserve_pages)
                        goto error;
                else
                        n -= totalreserve_pages;

                /*
                 * Leave the last 3% for root
                 */
                if (!cap_sys_admin)
                        n -= n / 32;
                free += n;

                if (free > pages)
                        return 0;

                goto error;
        }

        allowed = vm_commit_limit();
        /*
         * Reserve some for root
         */
        if (!cap_sys_admin)
                allowed -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);

        /* Don't let a single process grow too big:
           leave 3% of the size of this process for other processes */
        if (mm)
                allowed -= mm->total_vm / 32;

        if (percpu_counter_read_positive(&vm_committed_as) < allowed)
                return 0;
error:
        vm_unacct_memory(pages);

        return -ENOMEM;
}