linux-kernel - Re: [PATCH v4 16/17] maple_tree: remove unneeded mas_wr_walk() in mas_store

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Message-ID: <b4ad4b6a-f0b5-4ed1-a49a-d4e9ed0e4831@oracle.com>
Date: Fri, 25 Oct 2024 15:54:04 -0400
From: Sid Kumar <sidhartha.kumar@...cle.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: linux-kernel@...r.kernel.org, maple-tree@...ts.infradead.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org, liam.howlett@...cle.com,
        willy@...radead.org, surenb@...gle.com
Subject: Re: [PATCH v4 16/17] maple_tree: remove unneeded mas_wr_walk() in
 mas_store_prealloc()


On 10/23/24 9:20 PM, Wei Yang wrote:
> On Wed, Aug 14, 2024 at 12:19:43PM -0400, Sidhartha Kumar wrote:
>> Users of mas_store_prealloc() enter this function with nodes already
>> preallocated. This means the store type must be already set. We can then
>> remove the call to mas_wr_store_type() and initialize the write state to
>> continue the partial walk that was done when determining the store type.
>>
> May I ask what is the partial walk here means?
>
> It is the mas_wr_walk() in mas_wr_store_type() which is stopped because of it
> is spanning write?

Yes, this is what I meant by the partial walk that's already been 
started. It's the walk done by mas_wr_store_type().

> I may lost some background, so the assumption here is mas_wr_store_type() has
> already been invoked and the store type has been decided, right?

Ya users of mas_store_prealloc() should have already called 
mas_preallocate() which does:

     mas->store_type = mas_wr_store_type(&wr_mas);
     request = mas_prealloc_calc(&wr_mas, entry);

to set the store type and allocate the nodes.


>> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
>> ---
>> lib/maple_tree.c | 18 +++++++++++++-----
>> 1 file changed, 13 insertions(+), 5 deletions(-)
>>
>> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
>> index 8c1a1a483395..73ce63d9c3a0 100644
>> --- a/lib/maple_tree.c
>> +++ b/lib/maple_tree.c
>> @@ -3979,9 +3979,6 @@ static inline void mas_wr_end_piv(struct ma_wr_state *wr_mas)
>> 		wr_mas->end_piv = wr_mas->pivots[wr_mas->offset_end];
>> 	else
>> 		wr_mas->end_piv = wr_mas->mas->max;
>> -
>> -	if (!wr_mas->entry)
>> -		mas_wr_extend_null(wr_mas);
>> }
>>
>> static inline unsigned char mas_wr_new_end(struct ma_wr_state *wr_mas)
>> @@ -5532,8 +5529,19 @@ void mas_store_prealloc(struct ma_state *mas, void *entry)
>> {
>> 	MA_WR_STATE(wr_mas, mas, entry);
>>
>> -	mas_wr_prealloc_setup(&wr_mas);
>> -	mas_wr_store_type(&wr_mas);
>> +	if (mas->store_type == wr_store_root) {
>> +		mas_wr_prealloc_setup(&wr_mas);
>> +		goto store;
>> +	}
>> +
>> +	mas_wr_walk_descend(&wr_mas);
> This one does not descend the tree, just locate the offset in a node and
> adjust min/max. So not look like to continue the partial walk to me.
>
>> +	if (mas->store_type != wr_spanning_store) {
>> +		/* set wr_mas->content to current slot */
>> +		wr_mas.content = mas_slot_locked(mas, wr_mas.slots, mas->offset);
>> +		mas_wr_end_piv(&wr_mas);
> If not a spanning write, the previous walk should reach a leaf node, right?

Ya that's true.

> I am not sure why we don't need to check extend null here. Because we have
> already done it?


Ya we extend null in mas_wr_store_type() which has already been called 
at this point.


     /* At this point, we are at the leaf node that needs to be altered. */
     mas_wr_end_piv(wr_mas);
     if (!wr_mas->entry)
         mas_wr_extend_null(wr_mas);

Thanks,

Sid

>> +	}
>> +
>> +store:
>> 	trace_ma_write(__func__, mas, 0, entry);
>> 	mas_wr_store_entry(&wr_mas);
>> 	MAS_WR_BUG_ON(&wr_mas, mas_is_err(mas));
>> -- 
>> 2.46.0
>>