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Subjectkmem_cache_destroy: Can't free all objects (2.6)
Hello,

I'm working on performance testing netfilter ip_conntrack. The tested
machine is a dual Xeon PC with Serverworks chipset and e1000 cards.

"Calibrating" without ip_conntrack loaded in went fine. However as
the ip_conntrack module was removed after a test, sometimes I got the
message:

slab error in kmem_cache_destroy(): cache `ip_conntrack': Can't free all
objects
Call Trace:
[<c0136fe9>] kmem_cache_destroy+0xd8/0x116
[<f889f3ba>] ip_conntrack_cleanup+0x43/0x6c [ip_conntrack]
[<f889d61a>] init_or_cleanup+0x131/0x190 [ip_conntrack]
[<f889fc13>] fini+0xf/0x19 [ip_conntrack]
[<c012dd76>] sys_delete_module+0x13c/0x169
[<c01402ab>] sys_munmap+0x45/0x66
[<c0108f9b>] syscall_call+0x7/0xb

I cannot reliably reproduce the error. Sometimes it comes at the
first test run, sometimes at say the 20th, but I have never been able to
run all the planned 150 tests.

This happens both with 2.5.74 and 2.6.0-test1.

Rusty suggested to apply the following little patch to capture allocs and
frees:

--- linux-2.6.0-ct/mm/slab.c.orig 2003-07-25 13:48:26.000000000
+0200
+++ linux-2.6.0-ct/mm/slab.c 2003-07-25 13:56:51.000000000 +0200
@@ -280,6 +280,7 @@
struct list_head next;

/* 5) statistics */
+ atomic_t alloc,free;
#if STATS
unsigned long num_active;
unsigned long num_allocations;
@@ -1389,6 +1390,10 @@
up(&cache_chain_sem);

if (__cache_shrink(cachep)) {
+ printk("Slab cache %s: %u allocs vs %u frees\n",
+ cachep->name,
+ atomic_read(&cachep->alloc),
+ atomic_read(&cachep->free));
slab_error(cachep, "Can't free all objects");
down(&cache_chain_sem);
list_add(&cachep->next,&cache_chain);
@@ -2022,7 +2027,10 @@
*/
void * kmem_cache_alloc (kmem_cache_t *cachep, int flags)
{
- return __cache_alloc(cachep, flags);
+ void *ret = __cache_alloc(cachep, flags);
+ if (ret)
+ atomic_inc(&cachep->alloc);
+ return ret;
}

/**
@@ -2119,6 +2127,7 @@
{
unsigned long flags;

+ atomic_inc(&cachep->free);
local_irq_save(flags);
__cache_free(cachep, objp);
local_irq_restore(flags);
The result with the patch applied is as follows:

Slab cache ip_conntrack: 2052678 allocs vs 2052678 frees
slab error in kmem_cache_destroy(): cache `ip_conntrack': Can't free all objects
Call Trace:
[<c0133554>] __slab_error+0x24/0x28
[<c013426d>] kmem_cache_destroy+0xa5/0x128
[<f889f562>] ip_conntrack_cleanup+0x4a/0x70 [ip_conntrack]
[<f889d5fa>] init_or_cleanup+0x16a/0x170 [ip_conntrack]
[<f889fd63>] fini+0x7/0xb [ip_conntrack]
[<c012b75e>] sys_delete_module+0x14a/0x16c
[<c013da30>] sys_munmap+0x38/0x58
[<c0108c27>] syscall_call+0x7/0xb

Slab debugging is enabled, no single message in the log. /proc/slabinfo
says (long lines wrapped):

slabinfo - version: 2.0 (statistics)
# name <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab>
: tunables <batchcount> <limit <sharedfactor>
: slabdata <active_slabs> <num_slabs> <sharedavail>
: globalstat <listallocs> <maxobjs> <grown> <reaped> <error> <maxfreeable> <freelimit>
: cpustat <allochit <allocmiss <freehit <freemiss>
broken 128 2486 180 22 1
: tunables 32 16 8
: slabdata 113 113 128
: globalstat 664846 590238 26829 0 0 1 70
: cpustat 1 974008 78670 1990714 61964

Any hint or idea what can be the problem?

Please Cc answers to me, I'm not on the list.

Best regards,
Jozsef
-
E-mail : kadlec@blackhole.kfki.hu, kadlec@sunserv.kfki.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : KFKI Research Institute for Particle and Nuclear Physics
H-1525 Budapest 114, POB. 49, Hungary

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