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Date: Tue, 06 May 2008 17:03:15 +0200
From: Johannes Berg <johannes@...solutions.net>
To: netdev <netdev@...r.kernel.org>
Cc: Herbert Xu <herbert@...dor.apana.org.au>,
"David S. Miller" <davem@...emloft.net>
Subject: [DEBUG] truesize debugging
When I was digging into the truesize stuff and discussed with Herbert he
mentioned that we should really check the actual allocation size...
I have a number of questions and sort of todo items, but not really all
that much interest right now in pursuing them, so here they are:
1. why is sizeof(skb_shared_info) not accounted?
2. why do you think it's bad when somebody increases the skb size a
bit and uses the space (as indicated by the truesize bug print)
but not when the space is not used? The networking stack
*explicitly* allocates more space than would be necessary in
some places to avoid future expansions, but doesn't account for
that either. No truesize bugs are currently logged for that if
the space never ends up getting used (so since we don't see
truesize bugs I guess that optimisation to expand by at least
128 bytes sometimes is useless?)
3. why does pskb_expand_head not change truesize? right now, unless
the extra space isn't really used, I can't see how to use it at
all even on an skb that isn't accounted to a socket if that skb
can later ever be accounted to a socket.
4. the truesize debugging printout is pretty useless. maybe a patch
like the one below should be shipped in mainline that one can
optionally turn on when running into the problem frequently.
The patch below changes the truesize check to be more accurate and makes
the code print out a stacktrace from the last place the skb was
allocated, cloned or head-expanded.
johannes
---
include/linux/skbuff.h | 20 ++++++++++++++++++--
net/Kconfig | 9 +++++++++
net/core/skbuff.c | 34 ++++++++++++++++++++++++++++++++--
3 files changed, 59 insertions(+), 4 deletions(-)
--- everything.orig/include/linux/skbuff.h 2008-05-06 16:40:40.000000000 +0200
+++ everything/include/linux/skbuff.h 2008-05-06 16:43:08.000000000 +0200
@@ -28,6 +28,7 @@
#include <linux/rcupdate.h>
#include <linux/dmaengine.h>
#include <linux/hrtimer.h>
+#include <linux/stacktrace.h>
#define HAVE_ALLOC_SKB /* For the drivers to know */
#define HAVE_ALIGNABLE_SKB /* Ditto 8) */
@@ -188,6 +189,8 @@ enum {
#define NET_SKBUFF_DATA_USES_OFFSET 1
#endif
+#define NET_SKBUFF_STACKTRACE_ENTRIES 10
+
#ifdef NET_SKBUFF_DATA_USES_OFFSET
typedef unsigned int sk_buff_data_t;
#else
@@ -245,6 +248,8 @@ typedef unsigned char *sk_buff_data_t;
* @dma_cookie: a cookie to one of several possible DMA operations
* done by skb DMA functions
* @secmark: security marking
+ * @stacktrace: allocation stack trace
+ * @stacktracedata: allocation stack trace entries
*/
struct sk_buff {
@@ -321,6 +326,11 @@ struct sk_buff {
__u32 mark;
+#ifdef CONFIG_SKBUFF_ALLOC_TRACE
+ struct stack_trace stacktrace;
+ unsigned long stacktracedata[NET_SKBUFF_STACKTRACE_ENTRIES];
+#endif
+
sk_buff_data_t transport_header;
sk_buff_data_t network_header;
sk_buff_data_t mac_header;
@@ -387,9 +397,15 @@ extern void skb_truesize_bug(struc
static inline void skb_truesize_check(struct sk_buff *skb)
{
- int len = sizeof(struct sk_buff) + skb->len;
+#ifdef NET_SKBUFF_DATA_USES_OFFSET
+ int len = skb->end;
+#else
+ int len = (skb->end - skb->head);
+#endif
+
+ len += sizeof(struct sk_buff) + skb->data_len;
- if (unlikely((int)skb->truesize < len))
+ if (unlikely((int)skb->truesize != len))
skb_truesize_bug(skb);
}
--- everything.orig/net/core/skbuff.c 2008-05-06 16:40:40.000000000 +0200
+++ everything/net/core/skbuff.c 2008-05-06 17:02:07.000000000 +0200
@@ -151,9 +151,21 @@ void skb_under_panic(struct sk_buff *skb
void skb_truesize_bug(struct sk_buff *skb)
{
+#ifdef NET_SKBUFF_DATA_USES_OFFSET
+ int len = skb->end;
+#else
+ int len = (skb->end - skb->head);
+#endif
+
+ len += sizeof(struct sk_buff) + skb->data_len;
+
printk(KERN_ERR "SKB BUG: Invalid truesize (%u) "
- "len=%u, sizeof(sk_buff)=%Zd\n",
- skb->truesize, skb->len, sizeof(struct sk_buff));
+ "size=%u, sizeof(sk_buff)=%Zd\n",
+ skb->truesize, len, sizeof(struct sk_buff));
+#ifdef CONFIG_SKBUFF_ALLOC_TRACE
+ printk(KERN_DEBUG "last reallocate at:\n");
+ print_stack_trace(&skb->stacktrace, 0);
+#endif
}
EXPORT_SYMBOL(skb_truesize_bug);
@@ -219,6 +231,13 @@ struct sk_buff *__alloc_skb(unsigned int
shinfo->ip6_frag_id = 0;
shinfo->frag_list = NULL;
+#ifdef CONFIG_SKBUFF_ALLOC_TRACE
+ skb->stacktrace.max_entries = NET_SKBUFF_STACKTRACE_ENTRIES;
+ skb->stacktrace.entries = skb->stacktracedata;
+ skb->stacktrace.skip = 1;
+ save_stack_trace(&skb->stacktrace);
+#endif
+
if (fclone) {
struct sk_buff *child = skb + 1;
atomic_t *fclone_ref = (atomic_t *) (child + 1);
@@ -438,6 +457,12 @@ static void __copy_skb_header(struct sk_
#endif
#endif
skb_copy_secmark(new, old);
+#ifdef CONFIG_SKBUFF_ALLOC_TRACE
+ new->stacktrace.max_entries = NET_SKBUFF_STACKTRACE_ENTRIES;
+ new->stacktrace.entries = new->stacktracedata;
+ new->stacktrace.skip = 1;
+ save_stack_trace(&new->stacktrace);
+#endif
}
static struct sk_buff *__skb_clone(struct sk_buff *n, struct sk_buff *skb)
@@ -683,6 +708,11 @@ int pskb_expand_head(struct sk_buff *skb
if (!data)
goto nodata;
+#ifdef CONFIG_SKBUFF_ALLOC_TRACE
+ skb->stacktrace.max_entries = NET_SKBUFF_STACKTRACE_ENTRIES;
+ skb->stacktrace.nr_entries = 0;
+ save_stack_trace(&skb->stacktrace);
+#endif
/* Copy only real data... and, alas, header. This should be
* optimized for the cases when header is void. */
#ifdef NET_SKBUFF_DATA_USES_OFFSET
--- everything.orig/net/Kconfig 2008-05-06 16:40:40.000000000 +0200
+++ everything/net/Kconfig 2008-05-06 16:46:23.000000000 +0200
@@ -35,6 +35,15 @@ config NET_NS
Allow user space to create what appear to be multiple instances
of the network stack.
+config SKBUFF_ALLOC_TRACE
+ bool "SKB allocation stack tracking"
+ depends on EXPERIMENTAL && STACKTRACE_SUPPORT
+ select STACKTRACE
+ help
+ This option makes the skb allocation functions store a stack trace
+ into an SKB when allocated so that later one can dump it if
+ something is wrong with the SKB to find out where it came from.
+
source "net/packet/Kconfig"
source "net/unix/Kconfig"
source "net/xfrm/Kconfig"
--
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