Re: [PATCH drm-next v2 03/16] maple_tree: split up MA_STATE() macro

From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To: Danilo Krummrich <dakr@redhat.com>
Cc: airlied@gmail.com, daniel@ffwll.ch, tzimmermann@suse.de,
	mripard@kernel.org, corbet@lwn.net, christian.koenig@amd.com,
	bskeggs@redhat.com, matthew.brost@intel.com,
	boris.brezillon@collabora.com, alexdeucher@gmail.com,
	ogabbay@kernel.org, bagasdotme@gmail.com, willy@infradead.org,
	jason@jlekstrand.net, dri-devel@lists.freedesktop.org,
	nouveau@lists.freedesktop.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH drm-next v2 03/16] maple_tree: split up MA_STATE() macro
Date: Fri, 17 Feb 2023 13:34:04 -0500	[thread overview]
Message-ID: <20230217183404.jmjew5lrduts6cpo@revolver> (raw)
In-Reply-To: <20230217134422.14116-4-dakr@redhat.com>

* Danilo Krummrich <dakr@redhat.com> [230217 08:44]:
> Split up the MA_STATE() macro such that components using the maple tree
> can easily inherit from struct ma_state and build custom tree walk
> macros to hide their internals from users.
> 
> Example:
> 
> struct sample_iter {
> 	struct ma_state mas;
> 	struct sample_mgr *mgr;
> 	struct sample_entry *entry;
> };
> 
> \#define SAMPLE_ITER(name, __mgr) \
> 	struct sample_iter name = { \
> 		.mas = __MA_STATE(&(__mgr)->mt, 0, 0),
> 		.mgr = __mgr,
> 		.entry = NULL,
> 	}

I see this patch is to allow for anonymous maple states, this looks
good.

I've a lengthy comment about the iterator that I'm adding here to head
off anyone that may copy your example below.

> 
> \#define sample_iter_for_each_range(it__, start__, end__) \
> 	for ((it__).mas.index = start__, (it__).entry = mas_find(&(it__).mas, end__ - 1); \
> 	     (it__).entry; (it__).entry = mas_find(&(it__).mas, end__ - 1))

I see you've added something like the above in your patch set as well.
I'd like to point out that the index isn't the only state information
that needs to be altered here, and in fact, this could go very wrong.

The maple state has a node and an offset within that node.  If you set
the index to lower than the current position of your iterator and call
mas_find() then what happens is somewhat undefined.  I expect you will
get the wrong value (most likely either the current value or the very
next one that the iterator is already pointing to).  I believe you have
been using a fresh maple state for each iterator in your patches, but I
haven't had a deep look into your code yet.

We have methods of resetting the iterator and set the range (mas_set()
and mas_set_range()) which are safe for what you are doing, but they
will start the walk from the root node to the index again.

So, if you know what you are doing is safe, then the way you have
written it will work, but it's worth mentioning that this could occur.

It is also worth pointing out that it would be much safer to use a
function to do the above so you get type safety.. and I was asked to add
this to the VMA interface by Linus [1], which is on its way upstream [2].

1. https://lore.kernel.org/linux-mm/CAHk-=wg9WQXBGkNdKD2bqocnN73rDswuWsavBB7T-tekykEn_A@mail.gmail.com/
2. https://lore.kernel.org/linux-mm/20230120162650.984577-1-Liam.Howlett@oracle.com/

> 
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> ---
>  include/linux/maple_tree.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
> index e594db58a0f1..ca04c900e51a 100644
> --- a/include/linux/maple_tree.h
> +++ b/include/linux/maple_tree.h
> @@ -424,8 +424,8 @@ struct ma_wr_state {
>  #define MA_ERROR(err) \
>  		((struct maple_enode *)(((unsigned long)err << 2) | 2UL))
>  
> -#define MA_STATE(name, mt, first, end)					\
> -	struct ma_state name = {					\
> +#define __MA_STATE(mt, first, end)					\
> +	{								\
>  		.tree = mt,						\
>  		.index = first,						\
>  		.last = end,						\
> @@ -435,6 +435,9 @@ struct ma_wr_state {
>  		.alloc = NULL,						\
>  	}
>  
> +#define MA_STATE(name, mt, first, end)					\
> +	struct ma_state name = __MA_STATE(mt, first, end)
> +
>  #define MA_WR_STATE(name, ma_state, wr_entry)				\
>  	struct ma_wr_state name = {					\
>  		.mas = ma_state,					\
> -- 
> 2.39.1
> 