linux-kernel - Re: [PATCH v2 1/1] pgo: Fix allocate

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Date:   Thu, 3 Jun 2021 14:14:24 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Jarmo Tiitto <jarmo.tiitto@...il.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Bill Wendling <wcw@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Cc:     morbo@...gle.com
Subject: Re: [PATCH v2 1/1] pgo: Fix allocate_node() v2

On 6/3/2021 6:38 AM, Jarmo Tiitto 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.
> 
> 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@...il.com>

I agree with Nick on the comments about the commit message. A few more 
small nits below, not sure they necessitate a v3, up to you. Thank you 
for the patch!

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>   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>

Not sure that it actually matters but I think this should be

#include <asm/sections.h>

instead. Might be nice to keep this sorted by moving it to the top as well.

>   #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();

A blank line between this variable and the comment below would look nice.

> +	/* Check that p is within vmlinux __llvm_prf_data section. > +	 * If not, don't allocate since we can't handle modules yet.
> +	 */

For every subsystem except for netdev, there should be a blank line at 
the beginning of a comment. In other works:

/*
  * 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
>