linux-kernel - Re: [PATCH 1/3] maple_tree: Make maple state reusable after mas_empty_area

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Message-ID: <20230419225419.czzwbdgys367fax2@revolver>
Date:   Wed, 19 Apr 2023 18:54:19 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] maple_tree: Make maple state reusable after
 mas_empty_area_rev()

* Peng Zhang <zhangpeng.00@...edance.com> [230419 05:02]:
> 
> 在 2023/4/14 22:57, Liam R. Howlett 写道:
> > Stop using maple state min/max for the range by passing through pointers
> > for those values.  This will allow the maple state to be reused without
> > resetting.
> > 
> > Also add some logic to fail out early on searching with invalid
> > arguments.
> > 
> > Fixes: 54a611b60590 ("Maple Tree: add new data structure")
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > ---
> >   lib/maple_tree.c | 27 +++++++++++++--------------
> >   1 file changed, 13 insertions(+), 14 deletions(-)
> > 
> > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > index 4df6a0ce1c1b..ed350aa293b2 100644
> > --- a/lib/maple_tree.c
> > +++ b/lib/maple_tree.c
> > @@ -4938,7 +4938,8 @@ static inline void *mas_prev_entry(struct ma_state *mas, unsigned long min)
> >    * Return: True if found in a leaf, false otherwise.
> >    *
> >    */
> > -static bool mas_rev_awalk(struct ma_state *mas, unsigned long size)
> > +static bool mas_rev_awalk(struct ma_state *mas, unsigned long size,
> > +		unsigned long *gap_min, unsigned long *gap_max)
> >   {
> >   	enum maple_type type = mte_node_type(mas->node);
> >   	struct maple_node *node = mas_mn(mas);
> > @@ -5003,8 +5004,8 @@ static bool mas_rev_awalk(struct ma_state *mas, unsigned long size)
> >   	if (unlikely(ma_is_leaf(type))) {
> >   		mas->offset = offset;
> > -		mas->min = min;
> > -		mas->max = min + gap - 1;
> > +		*gap_min = min;
> > +		*gap_max = min + gap - 1;
> >   		return true;
> >   	}
> > @@ -5280,6 +5281,9 @@ int mas_empty_area(struct ma_state *mas, unsigned long min,
> >   	unsigned long *pivots;
> >   	enum maple_type mt;
> > +	if (min >= max)
> This can lead to errors, min == max is valid.
> I think it's better to change it to this:
> if (min > max || size == 0 || max - min < size - 1)

I am not sure what it means to search within a range of one.  I guess
you would expect it to just return that value if it's empty?

In any case, since we are dealing with pages of data for the VMAs,
having min == max really makes no sense, even with the subtraction of
one in the caller to reduce the max, the min and max should be at least
PAGE_SIZE - 1 apart here.

I think you are right, and I think this needs to be looked at for the
tree on its own, but I don't think it's a problem for the VMA user. I'll
write a testcase and ensure a search for a single entry in a single
entry window works separately.  Thanks for pointing this out.

> > +		return -EINVAL;
> > +
> >   	if (mas_is_start(mas))
> >   		mas_start(mas);
> >   	else if (mas->offset >= 2)
> > @@ -5334,6 +5338,9 @@ int mas_empty_area_rev(struct ma_state *mas, unsigned long min,
> >   {
> >   	struct maple_enode *last = mas->node;
> > +	if (min >= max)
> ditto.

I'll do the search in both directions.

> > +		return -EINVAL;
> > +
> >   	if (mas_is_start(mas)) {
> >   		mas_start(mas);
> >   		mas->offset = mas_data_end(mas);
> > @@ -5353,7 +5360,7 @@ int mas_empty_area_rev(struct ma_state *mas, unsigned long min,
> >   	mas->index = min;
> >   	mas->last = max;
> > -	while (!mas_rev_awalk(mas, size)) {
> > +	while (!mas_rev_awalk(mas, size, &min, &max)) {
> >   		if (last == mas->node) {
> >   			if (!mas_rewind_node(mas))
> >   				return -EBUSY;
> > @@ -5368,17 +5375,9 @@ int mas_empty_area_rev(struct ma_state *mas, unsigned long min,
> >   	if (unlikely(mas->offset == MAPLE_NODE_SLOTS))
> >   		return -EBUSY;
> > -	/*
> > -	 * mas_rev_awalk() has set mas->min and mas->max to the gap values.  If
> > -	 * the maximum is outside the window we are searching, then use the last
> > -	 * location in the search.
> > -	 * mas->max and mas->min is the range of the gap.
> > -	 * mas->index and mas->last are currently set to the search range.
> > -	 */
> > -
> >   	/* Trim the upper limit to the max. */
> > -	if (mas->max <= mas->last)
> > -		mas->last = mas->max;
> > +	if (max <= mas->last)
> > +		mas->last = max;
> 
> We can get max as follows, without using pointers to track min, max in
> mas_rev_awalk().
> 
> mt = mte_node_type(mas->node); pivots = ma_pivots(mas_mn(mas), mt); max =
> mas_logical_pivot(mas, pivots, mas->offset, mt);

Yes, but why would we do this?  We have done all this work already in
mas_rev_awalk(), and we have to do it there to get the offset in the
first place.

> 	if (max < mas->last) /* The equal sign here can be removed */

Thanks.  I'll keep this in mind when I revisit the function.  I don't
want to re-spin the patch for this alone.

> 		mas->last = max;
> 
> >   	mas->index = mas->last - size + 1;
> >   	return 0;