Re: [PATCH 1/3] maple_tree: Make maple state reusable after mas_empty_area_rev()

[Peng Zhang]

在 2023/4/20 06:54, Liam R. Howlett 写道:
> * Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [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@xxxxxxxxxx>
> > > ---
> > >    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?
Yes, if min==max, and the value pointed to by this index is empty,
we should return it.
> 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.
Yes, it's a problem with the tree itself.
> > > +		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.
Here is also the same problem with the tree itself as above.
Maybe I didn't understand what you mean. I think the case of
min==max should still be considered in mas_empty_area_rev(),
because mas_empty_area_rev() is a separate interface.
> > > +		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.
Yes, both methods will work.
> > 	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;