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Commit fb452a1a authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by David S. Miller
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Revert "net: use lib/percpu_counter API for fragmentation mem accounting" 



This reverts commit 6d7b857d.

There is a bug in fragmentation codes use of the percpu_counter API,
that can cause issues on systems with many CPUs.

The frag_mem_limit() just reads the global counter (fbc->count),
without considering other CPUs can have upto batch size (130K) that
haven't been subtracted yet.  Due to the 3MBytes lower thresh limit,
this become dangerous at >=24 CPUs (3*1024*1024/130000=24).

The correct API usage would be to use __percpu_counter_compare() which
does the right thing, and takes into account the number of (online)
CPUs and batch size, to account for this and call __percpu_counter_sum()
when needed.

We choose to revert the use of the lib/percpu_counter API for frag
memory accounting for several reasons:

1) On systems with CPUs > 24, the heavier fully locked
   __percpu_counter_sum() is always invoked, which will be more
   expensive than the atomic_t that is reverted to.

Given systems with more than 24 CPUs are becoming common this doesn't
seem like a good option.  To mitigate this, the batch size could be
decreased and thresh be increased.

2) The add_frag_mem_limit+sub_frag_mem_limit pairs happen on the RX
   CPU, before SKBs are pushed into sockets on remote CPUs.  Given
   NICs can only hash on L2 part of the IP-header, the NIC-RXq's will
   likely be limited.  Thus, a fair chance that atomic add+dec happen
   on the same CPU.

Revert note that commit 1d6119ba ("net: fix percpu memory leaks")
removed init_frag_mem_limit() and instead use inet_frags_init_net().
After this revert, inet_frags_uninit_net() becomes empty.

Fixes: 6d7b857d ("net: use lib/percpu_counter API for fragmentation mem accounting")
Fixes: 1d6119ba ("net: fix percpu memory leaks")
Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Acked-by: default avatarFlorian Westphal <fw@strlen.de>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 64327fc8

include/net/inet_frag.h

+9 −21
Original line number Original line Diff line number Diff line 
#ifndef __NET_FRAG_H__

#ifndef __NET_FRAG_H__

#define __NET_FRAG_H__

#define __NET_FRAG_H__





#include <linux/percpu_counter.h>



struct netns_frags {

struct netns_frags {

	/* The percpu_counter "mem" need to be cacheline aligned.

	/* Keep atomic mem on separate cachelines in structs that include it */

	 *  mem.count must not share cacheline with other writers

	atomic_t		mem ____cacheline_aligned_in_smp;

	 */

	struct percpu_counter   mem ____cacheline_aligned_in_smp;



	/* sysctls */

	/* sysctls */

	int			timeout;

	int			timeout;

	int			high_thresh;

	int			high_thresh;
@@ -110,11 +105,11 @@ void inet_frags_fini(struct inet_frags *); 




static inline int inet_frags_init_net(struct netns_frags *nf)

static inline int inet_frags_init_net(struct netns_frags *nf)

{

{

	return percpu_counter_init(&nf->mem, 0, GFP_KERNEL);

	atomic_set(&nf->mem, 0);

	return 0;

}

}

static inline void inet_frags_uninit_net(struct netns_frags *nf)

static inline void inet_frags_uninit_net(struct netns_frags *nf)

{

{

	percpu_counter_destroy(&nf->mem);

}

}





void inet_frags_exit_net(struct netns_frags *nf, struct inet_frags *f);

void inet_frags_exit_net(struct netns_frags *nf, struct inet_frags *f);
@@ -140,31 +135,24 @@ static inline bool inet_frag_evicting(struct inet_frag_queue *q) 




/* Memory Tracking Functions. */

/* Memory Tracking Functions. */





/* The default percpu_counter batch size is not big enough to scale to

 * fragmentation mem acct sizes.

 * The mem size of a 64K fragment is approx:

 *  (44 fragments * 2944 truesize) + frag_queue struct(200) = 129736 bytes

 */

static unsigned int frag_percpu_counter_batch = 130000;



static inline int frag_mem_limit(struct netns_frags *nf)

static inline int frag_mem_limit(struct netns_frags *nf)

{

{

	return percpu_counter_read(&nf->mem);

	return atomic_read(&nf->mem);

}

}





static inline void sub_frag_mem_limit(struct netns_frags *nf, int i)

static inline void sub_frag_mem_limit(struct netns_frags *nf, int i)

{

{

	percpu_counter_add_batch(&nf->mem, -i, frag_percpu_counter_batch);

	atomic_sub(i, &nf->mem);

}

}





static inline void add_frag_mem_limit(struct netns_frags *nf, int i)

static inline void add_frag_mem_limit(struct netns_frags *nf, int i)

{

{

	percpu_counter_add_batch(&nf->mem, i, frag_percpu_counter_batch);

	atomic_add(i, &nf->mem);

}

}





static inline unsigned int sum_frag_mem_limit(struct netns_frags *nf)

static inline int sum_frag_mem_limit(struct netns_frags *nf)

{

{

	return percpu_counter_sum_positive(&nf->mem);

	return atomic_read(&nf->mem);

}

}





/* RFC 3168 support :

/* RFC 3168 support :

net/ipv4/inet_fragment.c

+1 −3
Original line number Original line Diff line number Diff line
@@ -234,10 +234,8 @@ void inet_frags_exit_net(struct netns_frags *nf, struct inet_frags *f) 
	cond_resched();

	cond_resched();





	if (read_seqretry(&f->rnd_seqlock, seq) ||

	if (read_seqretry(&f->rnd_seqlock, seq) ||

	    percpu_counter_sum(&nf->mem))

	    sum_frag_mem_limit(nf))

		goto evict_again;

		goto evict_again;



	percpu_counter_destroy(&nf->mem);

}

}

EXPORT_SYMBOL(inet_frags_exit_net);

EXPORT_SYMBOL(inet_frags_exit_net);