linux-kernel - Re: [PATCH drm-next v2 03/16] maple_tree: split up MA

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Message-ID: <3c9e368a-0ecb-fc60-dac4-f12fa23d8f60@redhat.com>
Date:   Mon, 20 Feb 2023 14:48:34 +0100
From:   Danilo Krummrich <dakr@...hat.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>, airlied@...il.com,
        daniel@...ll.ch, tzimmermann@...e.de, mripard@...nel.org,
        corbet@....net, christian.koenig@....com, bskeggs@...hat.com,
        matthew.brost@...el.com, boris.brezillon@...labora.com,
        alexdeucher@...il.com, ogabbay@...nel.org, bagasdotme@...il.com,
        willy@...radead.org, jason@...kstrand.net,
        dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-next v2 03/16] maple_tree: split up MA_STATE() macro

On 2/17/23 19:34, Liam R. Howlett wrote:
> * Danilo Krummrich <dakr@...hat.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.

Yes, I'm aware that I'd need to reset the whole iterator in order to 
re-use it.

Regarding the other considerations of the iterator design please see my 
answer to Matthew.

> 
> 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/

You mean having wrappers like sample_find() instead of directly using 
mas_find()?

> 
>>
>> Signed-off-by: Danilo Krummrich <dakr@...hat.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
>>
>