netdev - Re: [BUG] 2.6.37-rc5 Memory leak in net/ipv4/udp.c

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Date:	Fri, 17 Dec 2010 12:47:22 +0100
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [BUG] 2.6.37-rc5 Memory leak in net/ipv4/udp.c

Eric Dumazet writes:
> Le vendredi 17 décembre 2010 à 12:11 +0100, Lothar Waßmann a écrit :
> > Hi,
> > 
> > Eric Dumazet writes:
> > > Le vendredi 17 décembre 2010 à 11:18 +0100, Lothar Waßmann a écrit :
> > > > The offending code in net/ipv4/udp.c is:
> > > > |void __init udp_table_init(struct udp_table *table, const char *name)
> > > > |{
> > > > |	unsigned int i;
> > > > |
> > > > |	if (!CONFIG_BASE_SMALL)
> > > > |		table->hash = alloc_large_system_hash(name,
> > > > |			2 * sizeof(struct udp_hslot),
> > > > |			uhash_entries,
> > > > |			21, /* one slot per 2 MB */
> > > > |			0,
> > > > |			&table->log,
> > > > |			&table->mask,
> > > > |			64 * 1024);
> > > > |	/*
> > > > |	 * Make sure hash table has the minimum size
> > > > |	 */
> > > > |	if (CONFIG_BASE_SMALL || table->mask < UDP_HTABLE_SIZE_MIN - 1) {
> > > > |		table->hash = kmalloc(UDP_HTABLE_SIZE_MIN *
> > > > |				      2 * sizeof(struct udp_hslot), GFP_KERNEL);
> > > > In case of !CONFIG_BASE_SMALL and 'table->mask < UDP_HTABLE_SIZE_MIN - 1)'
> > > > the memory allocated in the previous if clause becomes inacessible!
> > > > 
> > > > Shouldn't this be:
> > > > |	if (!CONFIG_BASE_SMALL && table->mask >= UDP_HTABLE_SIZE_MIN - 1) {
> > > > |		table->hash = alloc_large_system_hash(name,
> > > > |			2 * sizeof(struct udp_hslot),
> > > > |			uhash_entries,
> > > > |			21, /* one slot per 2 MB */
> > > > |			0,
> > > > |			&table->log,
> > > > |			&table->mask,
> > > > |			64 * 1024);
> > > > |	} else {
> > > > |		table->hash = kmalloc(UDP_HTABLE_SIZE_MIN *
> > > > |				      2 * sizeof(struct udp_hslot), GFP_KERNEL);
> > > > [...]
> > > > 
> > > 
> > > Nothing we can do about it, there is no API to reverse the
> > > alloc_large_system_hash() effect. We could call kmemleak api to at least
> > > avoid this false alarm.
> > > 
> > Do you have to call it at all in case of table->mask < UDP_HTABLE_SIZE_MIN - 1?
> > 
> 
> We call alloc_large_system_hash() asking it to size the table _itself_.
> We give some hints : 
> 
> - How many slots per MB of avail memory.
> - An upper limit (64*1024 slots because we only handle 65536 udp ports)
> - but not a lower limit (not available in the API)
> 
> Problem is in your case, alloc_large_system_hash() allocates a very
> small area. Then we catch the problem, seeing table->mask is too small
> for our needs. We prefer to 'lost' this too small memory than crashing
> kernel later.
> 
table->mask is not altered by alloc_large_system_hash(), so you could
detect the situation beforhand and avoid calling that function in this
case. As far as I can tell there is no need for
alloc_large_system_hash() if you later decide to use kmalloc'ed memory
instead.

The current situation is
if (!CONFIG_BASE_SMALL)
	call alloc_large_system_hash()
if (CONFIG_BASE_SMALL || table->mask < MIN)
	call kmalloc() dropping evnetually allocated memory from the
	previous if clause

My proposal was:
if (!CONFIG_BASE_SMALL && table->mask >= MIN)
	call alloc_large_system_hash()
else
	call kmalloc()

which is functionally equivalent except for the missing call to
alloc_large_system_hash() if the memory allocated by that function is
not used.

> > > We really want a minimum size for the UDP hash table, because our algos
> > > depend on this.
> > > 
> > I can't see why this could not be achieved by doing _either_
> > alloc_large_system_hash() _OR_ kmalloc() as stated above, but not
> > both.
> 
> We definitly want alloc_large_system_hash() for the general case
> (nice NUMA spread, while kmalloc() would allocate the hash table on a
> single memory node. Not so nice)
> 
That would still be the case with my proposed solution.


Lothar Waßmann
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