Re: [PATCH v2 1/1] pgo: Fix allocate_node() v2

From: Nathan Chancellor
Date: Thu Jun 03 2021 - 16:52:46 EST

On 6/3/2021 1:50 PM, Nick Desaulniers wrote:
On Thu, Jun 3, 2021 at 6:41 AM Jarmo Tiitto <jarmo.tiitto@xxxxxxxxx> wrote:

Based on Kees and others feedback here is v2 patch
that clarifies why the current checks in allocate_node()
are flawed. I did fair amount of KGDB time on it.

Kees can probably cut it when merging, but the above paragraph might
be better "below the fold" below next time (doesn't necessitate a v3).

When clang instrumentation eventually calls allocate_node()
the struct llvm_prf_data *p argument tells us from what section
we should reserve the vnode: It either points into vmlinux's
core __llvm_prf_data section or some loaded module's
__llvm_prf_data section.

But since we don't have access to corresponding
__llvm_prf_vnds section(s) for any module, the function
should return just NULL and ignore any profiling attempts
from modules for now.

Signed-off-by: Jarmo Tiitto <jarmo.tiitto@xxxxxxxxx>

^ ie. here. If you put text between the `---` and the diffstat, git
just discards it when applying. It's a good way to hide commentary
just meant for reviewers when sending a patch.

kernel/pgo/instrument.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
index 0e07ee1b17d9..afe9982b07a3 100644
--- a/kernel/pgo/instrument.c
+++ b/kernel/pgo/instrument.c
@@ -23,6 +23,7 @@
#include <linux/export.h>
#include <linux/spinlock.h>
#include <linux/types.h>
+#include <asm-generic/sections.h>
#include "pgo.h"

@@ -55,17 +56,19 @@ void prf_unlock(unsigned long flags)
static struct llvm_prf_value_node *allocate_node(struct llvm_prf_data *p,
u32 index, u64 value)
- if (&__llvm_prf_vnds_start[current_node + 1] >= __llvm_prf_vnds_end)
- return NULL; /* Out of nodes */
- current_node++;
- /* Make sure the node is entirely within the section */
- if (&__llvm_prf_vnds_start[current_node] >= __llvm_prf_vnds_end ||
- &__llvm_prf_vnds_start[current_node + 1] > __llvm_prf_vnds_end)
+ const int max_vnds = prf_vnds_count();

Sorry, where was prf_vnds_count() defined? I don't see it in
linux-next, but I'm also not sure which tree has

It is generated via the __DEFINE_PRF_SIZE macro in kernel/pgo/pgo.h.

+ /* Check that p is within vmlinux __llvm_prf_data section.
+ * If not, don't allocate since we can't handle modules yet.
+ */
+ if (!memory_contains(__llvm_prf_data_start,
+ __llvm_prf_data_end, p, sizeof(*p)))
return NULL;

- return &__llvm_prf_vnds_start[current_node];
+ if (WARN_ON_ONCE(current_node >= max_vnds))
+ return NULL; /* Out of nodes */
+ /* reserve vnode for vmlinux */
+ return &__llvm_prf_vnds_start[current_node++];


base-commit: 5d0cda65918279ada060417c5fecb7e86ccb3def

~Nick Desaulniers