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

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Message-ID: <20241025235850.5o6l3k2u2so6hxrs@master>
Date: Fri, 25 Oct 2024 23:58:50 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Sid Kumar <sidhartha.kumar@...cle.com>
Cc: Wei Yang <richard.weiyang@...il.com>, 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 Fri, Oct 25, 2024 at 03:54:04PM -0400, Sid Kumar wrote:
>
>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,

Hmm... if we have already done this, why we need to do mas_wr_end_piv() again?

>
>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
>> > 

-- 
Wei Yang
Help you, Help me