linux-kernel - Re: [PATCH] stackdepot: Rename pool_index to pool_index_plus

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Date: Tue, 2 Apr 2024 08:54:28 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Peter Collingbourne <pcc@...gle.com>,
 Andrey Konovalov <andreyknvl@...il.com>, Oscar Salvador <osalvador@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Michal Hocko <mhocko@...e.com>,
 Marco Elver <elver@...gle.com>, Alexander Potapenko <glider@...gle.com>,
 Omar Sandoval <osandov@...com>
Subject: Re: [PATCH] stackdepot: Rename pool_index to pool_index_plus_1

On 4/2/24 2:14 AM, Peter Collingbourne wrote:
> Commit 3ee34eabac2a ("lib/stackdepot: fix first entry having a
> 0-handle") changed the meaning of the pool_index field to mean "the
> pool index plus 1". This made the code accessing this field less
> self-documenting, as well as causing debuggers such as drgn to not
> be able to easily remain compatible with both old and new kernels,
> because they typically do that by testing for presence of the new
> field. Because stackdepot is a debugging tool, we should make sure
> that it is debugger friendly. Therefore, give the field a different
> name to improve readability as well as enabling debugger backwards
> compatibility.
> 
> Signed-off-by: Peter Collingbourne <pcc@...gle.com>
> Link: https://linux-review.googlesource.com/id/Ib3e70c36c1d230dd0a118dc22649b33e768b9f88

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
> Although this technically isn't a bug fix in the kernel,
> I would appreciate if this could be picked up for 6.9
> to avoid temporarily introducing a silent regression in drgn
> (loud regressions are fine).
> 
>  include/linux/stackdepot.h | 7 +++----
>  lib/stackdepot.c           | 4 ++--
>  2 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
> index 3c6caa5abc7c..e9ec32fb97d4 100644
> --- a/include/linux/stackdepot.h
> +++ b/include/linux/stackdepot.h
> @@ -44,10 +44,9 @@ typedef u32 depot_stack_handle_t;
>  union handle_parts {
>  	depot_stack_handle_t handle;
>  	struct {
> -		/* pool_index is offset by 1 */
> -		u32 pool_index	: DEPOT_POOL_INDEX_BITS;
> -		u32 offset	: DEPOT_OFFSET_BITS;
> -		u32 extra	: STACK_DEPOT_EXTRA_BITS;
> +		u32 pool_index_plus_1	: DEPOT_POOL_INDEX_BITS;
> +		u32 offset		: DEPOT_OFFSET_BITS;
> +		u32 extra		: STACK_DEPOT_EXTRA_BITS;
>  	};
>  };
>  
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index af6cc19a2003..68c97387aa54 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -330,7 +330,7 @@ static struct stack_record *depot_pop_free_pool(void **prealloc, size_t size)
>  	stack = current_pool + pool_offset;
>  
>  	/* Pre-initialize handle once. */
> -	stack->handle.pool_index = pool_index + 1;
> +	stack->handle.pool_index_plus_1 = pool_index + 1;
>  	stack->handle.offset = pool_offset >> DEPOT_STACK_ALIGN;
>  	stack->handle.extra = 0;
>  	INIT_LIST_HEAD(&stack->hash_list);
> @@ -441,7 +441,7 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
>  	const int pools_num_cached = READ_ONCE(pools_num);
>  	union handle_parts parts = { .handle = handle };
>  	void *pool;
> -	u32 pool_index = parts.pool_index - 1;
> +	u32 pool_index = parts.pool_index_plus_1 - 1;
>  	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
>  	struct stack_record *stack;
>