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 22:36:52 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org
CC:	dhowells@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MM : alloc_large_system_hash() can free some memory for
 non power-of-two bucketsize

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Sat, 19 May 2007 20:07:11 +0200
> 
>> Maybe David has an idea how this can be done properly ?
>>
>> ref : http://marc.info/?l=linux-netdev&m=117706074825048&w=2
> 
> You need to use __GFP_COMP or similar to make this splitting+freeing
> thing work.
> 
> Otherwise the individual pages don't have page references, only
> the head page of the high-order page will.
> 

Oh thanks David for the hint.

I added a split_page() call and it seems to work now.

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

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.

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>

View attachment "alloc_large.patch" of type "text/plain" (824 bytes)