code cat

dentry cache hash table size 지정은,
fs/dcache.c에서 alloc_large_system_hash를 통해서 된다.  그전에 dhash_entries는 kernel cmdline에서 지정하여 원하는 값을 패스 할 수 있다.
static void __init dcache_init(void)
{
        int loop;

        /* 
         * A constructor could be added for stable state like the lists,
         * but it is probably not worth it because of the cache nature
         * of the dcache. 
         */
        dentry_cache = KMEM_CACHE(dentry,
                SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_MEM_SPREAD);

        /* Hash may have been set up in dcache_init_early */
        if (!hashdist)
                return;

        dentry_hashtable =
                alloc_large_system_hash("Dentry cache",
                                        sizeof(struct hlist_bl_head),
                                        dhash_entries,
                                        13,
                                        0,
                                        &d_hash_shift,
                                        &d_hash_mask,
                                        0);

        for (loop = 0; loop < (1 << d_hash_shift); loop++)
                INIT_HLIST_BL_HEAD(dentry_hashtable + loop);
}

그리고 만약 dhash_entries값이 0이면(전역비초기화 변수 이므로 0으로 되어 bss영역에 있을 것이다) 5395라인 하위로 타고 들어가, nr_kernel_pages에 기반을 둔 2의 배수를 설정하고 기타 log2작업(아 이건 나중에 보자...)을 통해 hash table size를 출력한다. 

void *__init alloc_large_system_hash(const char *tablename,
                                     unsigned long bucketsize,
                                     unsigned long numentries,
                                     int scale,
                                     int flags,
                                     unsigned int *_hash_shift,
                                     unsigned int *_hash_mask,
                                     unsigned long limit)
{
        unsigned long long max = limit;
        unsigned long log2qty, size;
        void *table = NULL;

        /* allow the kernel cmdline to have a say */
        if (!numentries) {
                /* round applicable memory size up to nearest megabyte */
                numentries = nr_kernel_pages;
                numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
                numentries >>= 20 - PAGE_SHIFT;
                numentries <<= 20 - PAGE_SHIFT;

                /* limit to 1 bucket per 2^scale bytes of low memory */
                if (scale > PAGE_SHIFT)
                        numentries >>= (scale - PAGE_SHIFT);
                else
                        numentries <<= (PAGE_SHIFT - scale);

                /* Make sure we've got at least a 0-order allocation.. */
                if (unlikely(flags & HASH_SMALL)) {
                        /* Makes no sense without HASH_EARLY */
                        WARN_ON(!(flags & HASH_EARLY));
                        if (!(numentries >> *_hash_shift)) {
                                numentries = 1UL << *_hash_shift;
                                BUG_ON(!numentries);
                        }
                } else if (unlikely((numentries * bucketsize) < PAGE_SIZE))
                        numentries = PAGE_SIZE / bucketsize;
        }
        numentries = roundup_pow_of_two(numentries);

        /* limit allocation size to 1/16 total memory by default */
        if (max == 0) {
                max = ((unsigned long long)nr_all_pages << PAGE_SHIFT) >> 4;
                do_div(max, bucketsize);
        }

        if (numentries > max)
                numentries = max;

        log2qty = ilog2(numentries);

        do {
                size = bucketsize << log2qty;
                if (flags & HASH_EARLY)
                        table = alloc_bootmem_nopanic(size);
                else if (hashdist)
                        table = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL);
                else {
                        /*
                         * If bucketsize is not a power-of-two, we may free
                         * some pages at the end of hash table which
                         * alloc_pages_exact() automatically does
                         */
                        if (get_order(size) < MAX_ORDER) {
                                table = alloc_pages_exact(size, GFP_ATOMIC);
                                kmemleak_alloc(table, size, 1, GFP_ATOMIC);
                        }
                }
        } while (!table && size > PAGE_SIZE && --log2qty);

        if (!table)
                panic("Failed to allocate %s hash table\n", tablename);

        printk(KERN_INFO "%s hash table entries: %ld (order: %d, %lu bytes)\n",
               tablename,
               (1UL << log2qty),
               ilog2(size) - PAGE_SHIFT,
               size);

        if (_hash_shift)
                *_hash_shift = log2qty;
        if (_hash_mask)
                *_hash_mask = (1 << log2qty) - 1;

        return table;
}