linux-kernel - Re: [PATCH] MM : alloc_large_system_hash() can free some memory for non power-of-two bucketsize

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Date:	Sat, 19 May 2007 20:07:11 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] MM : alloc_large_system_hash() can free some memory for
 non power-of-two bucketsize

Andrew Morton a écrit :
> On Fri, 18 May 2007 11:54:54 +0200 Eric Dumazet <dada1@...mosbay.com> wrote:
> 
>> alloc_large_system_hash() is called at boot time to allocate space for several large hash tables.
>>
>> Lately, TCP hash table was changed and its bucketsize is not a power-of-two anymore.
>>
>> On most setups, alloc_large_system_hash() allocates one big page (order > 0) with __get_free_pages(GFP_ATOMIC, order). This single high_order page has a power-of-two size, bigger than the needed size.
> 
> Watch the 200-column text, please.
> 
>> We can free all pages that wont be used by the hash table.
>>
>> On a 1GB i386 machine, this patch saves 128 KB of LOWMEM memory.
>>
>> TCP established hash table entries: 32768 (order: 6, 393216 bytes)
>>
>> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
>> ---
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index ae96dd8..2e0ba08 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3350,6 +3350,20 @@ void *__init alloc_large_system_hash(const char *tablename,
>>  			for (order = 0; ((1UL << order) << PAGE_SHIFT) < size; order++)
>>  				;
>>  			table = (void*) __get_free_pages(GFP_ATOMIC, order);
>> +			/*
>> +			 * If bucketsize is not a power-of-two, we may free
>> +			 * some pages at the end of hash table.
>> +			 */
>> +			if (table) {
>> +				unsigned long alloc_end = (unsigned long)table +
>> +						(PAGE_SIZE << order);
>> +				unsigned long used = (unsigned long)table +
>> +						PAGE_ALIGN(size);
>> +				while (used < alloc_end) {
>> +					free_page(used);
>> +					used += PAGE_SIZE;
>> +				}
>> +			}
>>  		}
>>  	} while (!table && size > PAGE_SIZE && --log2qty);
>>  
> 
> It went BUG.
> 
> static inline int put_page_testzero(struct page *page)
> {
> 	VM_BUG_ON(atomic_read(&page->_count) == 0);
> 	return atomic_dec_and_test(&page->_count);
> }
> 
> http://userweb.kernel.org/~akpm/s5000523.jpg
> http://userweb.kernel.org/~akpm/config-vmm.txt

I see :(

Maybe David has an idea how this can be done properly ?

ref : http://marc.info/?l=linux-netdev&m=117706074825048&w=2


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