Date:   Mon, 11 Sep 2023 09:36:50 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     corbet@....net, akpm@...ux-foundation.org, willy@...radead.org,
        brauner@...nel.org, surenb@...gle.com, michael.christie@...cle.com,
        peterz@...radead.org, mathieu.desnoyers@...icios.com,
        npiggin@...il.com, avagin@...il.com, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] maple_tree: Introduce interfaces __mt_dup() and
 mtree_dup()

* Peng Zhang <zhangpeng.00@...edance.com> [230911 08:59]:
> 
> 
> 在 2023/9/8 04:13, Liam R. Howlett 写道:
> > * Peng Zhang <zhangpeng.00@...edance.com> [230830 08:57]:
> > > Introduce interfaces __mt_dup() and mtree_dup(), which are used to
> > > duplicate a maple tree. Compared with traversing the source tree and
> > > reinserting entry by entry in the new tree, it has better performance.
> > > The difference between __mt_dup() and mtree_dup() is that mtree_dup()
> > > handles locks internally.
> > 
> > __mt_dup() should be called mas_dup() to indicate the advanced interface
> > which requires users to handle their own locks.
> Changing to the mas_dup() interface may look like this:
> mas_dup(mas_old, mas_new)
> 
> This still encounters the problem we discussed before. You expect both
> mas_old and mas_new to point to the first element after the function
> returns, but for_each_vma(vmi, mpnt) in dup_mmap() does not support
> this, and will skip the first element.
> 
> Unless we have an iterator similar to "do {} while()", we have to reset
> mas_new. There is still additional overhead in making both mas_old and
> mas_new point to the first element, because mas will point to the last
> node after dfs order traversal.

I was only looking for the name change.  Although, I think we could have
written in a way to avoid skipping the first element.

> 
> In fact, I think mtree_dup() and __mt_dup() are enough. They seem to
> match mtree_destroy() and __mt_destroy() very well. Underlines indicate
> that users need to handle the lock themselves.

I think you are correct, __mt_dup() doesn't take a maple state.  Thanks
for pointing that out.  Please leave it the way you have it.

> > 
> > > 
> > > Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> > > ---
> > >   include/linux/maple_tree.h |   3 +
> > >   lib/maple_tree.c           | 265 +++++++++++++++++++++++++++++++++++++
> > >   2 files changed, 268 insertions(+)
> > > 
> > > diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
> > > index e41c70ac7744..44fe8a57ecbd 100644
> > > --- a/include/linux/maple_tree.h
> > > +++ b/include/linux/maple_tree.h
> > > @@ -327,6 +327,9 @@ int mtree_store(struct maple_tree *mt, unsigned long index,
> > >   		void *entry, gfp_t gfp);
> > >   void *mtree_erase(struct maple_tree *mt, unsigned long index);
> > > +int mtree_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp);
> > > +int __mt_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp);
> > > +
> > >   void mtree_destroy(struct maple_tree *mt);
> > >   void __mt_destroy(struct maple_tree *mt);
> > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > index ef234cf02e3e..8f841682269c 100644
> > > --- a/lib/maple_tree.c
> > > +++ b/lib/maple_tree.c
> > > @@ -6370,6 +6370,271 @@ void *mtree_erase(struct maple_tree *mt, unsigned long index)
> > >   }
> > >   EXPORT_SYMBOL(mtree_erase);
> > > +/*
> > > + * mas_dup_free() - Free a half-constructed tree.
> > 
> > Maybe "Free an incomplete duplication of a tree" ?
> > 
> > > + * @mas: Points to the last node of the half-constructed tree.
> > 
> > Your use of "Points to" seems to indicate someone knows you are talking
> > about a "maple state that has a node pointing to".  Can this be made
> > more clear?
> > @mas: The maple state of a incomplete tree.
> > 
> > Then add a note that @mas->node points to the last successfully
> > allocated node?
> > 
> > Or something along those lines.
> > 
> > > + *
> > > + * This function frees all nodes starting from @mas->node in the reverse order
> > > + * of mas_dup_build(). There is no need to hold the source tree lock at this
> > > + * time.
> > > + */
> > > +static void mas_dup_free(struct ma_state *mas)
> > > +{
> > > +	struct maple_node *node;
> > > +	enum maple_type type;
> > > +	void __rcu **slots;
> > > +	unsigned char count, i;
> > > +
> > > +	/* Maybe the first node allocation failed. */
> > > +	if (!mas->node)
> > > +		return;
> > > +
> > > +	while (!mte_is_root(mas->node)) {
> > > +		mas_ascend(mas);
> > > +
> > > +		if (mas->offset) {
> > > +			mas->offset--;
> > > +			do {
> > > +				mas_descend(mas);
> > > +				mas->offset = mas_data_end(mas);
> > > +			} while (!mte_is_leaf(mas->node));
> > 
> > Can you blindly descend and check !mte_is_leaf()?  What happens when the
> > tree duplication fails at random internal nodes?  Maybe I missed how
> > this cannot happen?
> > 
> > > +
> > > +			mas_ascend(mas);
> > > +		}
> > > +
> > > +		node = mte_to_node(mas->node);
> > > +		type = mte_node_type(mas->node);
> > > +		slots = (void **)ma_slots(node, type);
> > > +		count = mas_data_end(mas) + 1;
> > > +		for (i = 0; i < count; i++)
> > > +			((unsigned long *)slots)[i] &= ~MAPLE_NODE_MASK;
> > > +
> > > +		mt_free_bulk(count, slots);
> > > +	}
> > 
> > 
> > > +
> > > +	node = mte_to_node(mas->node);
> > > +	mt_free_one(node);
> > > +}
> > > +
> > > +/*
> > > + * mas_copy_node() - Copy a maple node and allocate child nodes.
> > 
> > if required. "..and allocate child nodes if required."
> > 
> > > + * @mas: Points to the source node.
> > > + * @new_mas: Points to the new node.
> > > + * @parent: The parent node of the new node.
> > > + * @gfp: The GFP_FLAGS to use for allocations.
> > > + *
> > > + * Copy @mas->node to @new_mas->node, set @parent to be the parent of
> > > + * @new_mas->node and allocate new child nodes for @new_mas->node.
> > > + * If memory allocation fails, @mas is set to -ENOMEM.
> > > + */
> > > +static inline void mas_copy_node(struct ma_state *mas, struct ma_state *new_mas,
> > > +		struct maple_node *parent, gfp_t gfp)
> > > +{
> > > +	struct maple_node *node = mte_to_node(mas->node);
> > > +	struct maple_node *new_node = mte_to_node(new_mas->node);
> > > +	enum maple_type type;
> > > +	unsigned long val;
> > > +	unsigned char request, count, i;
> > > +	void __rcu **slots;
> > > +	void __rcu **new_slots;
> > > +
> > > +	/* Copy the node completely. */
> > > +	memcpy(new_node, node, sizeof(struct maple_node));
> > > +
> > > +	/* Update the parent node pointer. */
> > > +	if (unlikely(ma_is_root(node)))
> > > +		val = MA_ROOT_PARENT;
> > > +	else
> > > +		val = (unsigned long)node->parent & MAPLE_NODE_MASK;
> > 
> > If you treat the root as special and outside the loop, then you can
> > avoid the check for root for every non-root node.  For root, you just
> > need to copy and do this special parent thing before the main loop in
> > mas_dup_build().  This will avoid an extra branch for each VMA over 14,
> > so that would add up to a lot of instructions.
> > 
> > > +
> > > +	new_node->parent = ma_parent_ptr(val | (unsigned long)parent);
> > > +
> > > +	if (mte_is_leaf(mas->node))
> > > +		return;
> > 
> > You are checking here and in mas_dup_build() for the leaf, splitting the
> > function into parent assignment and allocate would allow you to check
> > once. Copy could be moved to the main loop or with the parent setting,
> > depending on how you handle the root suggestion above.
> > 
> > > +
> > > +	/* Allocate memory for child nodes. */
> > > +	type = mte_node_type(mas->node);
> > > +	new_slots = ma_slots(new_node, type);
> > > +	request = mas_data_end(mas) + 1;
> > > +	count = mt_alloc_bulk(gfp, request, new_slots);
> > > +	if (unlikely(count < request)) {
> > > +		if (count)
> > > +			mt_free_bulk(count, new_slots);
> > 
> > The new_slots will still contain the addresses of the freed nodes.
> > Don't you need to clear it here to avoid a double free?  Is there a
> > test case for this in your testing?  Again, I may have missed how this
> > is not possible..
> > 
> > > +		mas_set_err(mas, -ENOMEM);
> > > +		return;
> > > +	}
> > > +
> > > +	/* Restore node type information in slots. */
> > > +	slots = ma_slots(node, type);
> > > +	for (i = 0; i < count; i++)
> > > +		((unsigned long *)new_slots)[i] |=
> > > +			((unsigned long)mt_slot_locked(mas->tree, slots, i) &
> > > +			MAPLE_NODE_MASK);
> > 
> > Can you expand this to multiple lines to make it more clear what is
> > going on?
> > 
> > > +}
> > > +
> > > +/*
> > > + * mas_dup_build() - Build a new maple tree from a source tree
> > > + * @mas: The maple state of source tree.
> > > + * @new_mas: The maple state of new tree.
> > > + * @gfp: The GFP_FLAGS to use for allocations.
> > > + *
> > > + * This function builds a new tree in DFS preorder. If the memory allocation
> > > + * fails, the error code -ENOMEM will be set in @mas, and @new_mas points to the
> > > + * last node. mas_dup_free() will free the half-constructed tree.
> > > + *
> > > + * Note that the attributes of the two trees must be exactly the same, and the
> > > + * new tree must be empty, otherwise -EINVAL will be returned.
> > > + */
> > > +static inline void mas_dup_build(struct ma_state *mas, struct ma_state *new_mas,
> > > +		gfp_t gfp)
> > > +{
> > > +	struct maple_node *node, *parent;
> > 
> > Could parent be struct maple_pnode?
> > 
> > > +	struct maple_enode *root;
> > > +	enum maple_type type;
> > > +
> > > +	if (unlikely(mt_attr(mas->tree) != mt_attr(new_mas->tree)) ||
> > > +	    unlikely(!mtree_empty(new_mas->tree))) {
> > > +		mas_set_err(mas, -EINVAL);
> > > +		return;
> > > +	}
> > > +
> > > +	mas_start(mas);
> > > +	if (mas_is_ptr(mas) || mas_is_none(mas)) {
> > > +		/*
> > > +		 * The attributes of the two trees must be the same before this.
> > > +		 * The following assignment makes them the same height.
> > > +		 */
> > > +		new_mas->tree->ma_flags = mas->tree->ma_flags;
> > > +		rcu_assign_pointer(new_mas->tree->ma_root, mas->tree->ma_root);
> > > +		return;
> > > +	}
> > > +
> > > +	node = mt_alloc_one(gfp);
> > > +	if (!node) {
> > > +		new_mas->node = NULL;
> > 
> > We don't have checks around for node == NULL, MAS_NONE would be a safer
> > choice.  It is unlikely that someone would dup the tree and fail then
> > call something else, but I avoid setting node to NULL.
> > 
> > > +		mas_set_err(mas, -ENOMEM);
> > > +		return;
> > > +	}
> > > +
> > > +	type = mte_node_type(mas->node);
> > > +	root = mt_mk_node(node, type);
> > > +	new_mas->node = root;
> > > +	new_mas->min = 0;
> > > +	new_mas->max = ULONG_MAX;
> > > +	parent = ma_mnode_ptr(new_mas->tree);
> > > +
> > > +	while (1) {
> > > +		mas_copy_node(mas, new_mas, parent, gfp);
> > > +
> > > +		if (unlikely(mas_is_err(mas)))
> > > +			return;
> > > +
> > > +		/* Once we reach a leaf, we need to ascend, or end the loop. */
> > > +		if (mte_is_leaf(mas->node)) {
> > > +			if (mas->max == ULONG_MAX) {
> > > +				new_mas->tree->ma_flags = mas->tree->ma_flags;
> > > +				rcu_assign_pointer(new_mas->tree->ma_root,
> > > +						   mte_mk_root(root));
> > > +				break;
> > 
> > If you move this to the end of the function, you can replace the same
> > block above with a goto.  That will avoid breaking the line up.
> > 
> > > +			}
> > > +
> > > +			do {
> > > +				/*
> > > +				 * Must not at the root node, because we've
> > > +				 * already end the loop when we reach the last
> > > +				 * leaf.
> > > +				 */
> > 
> > I'm not sure what the comment above is trying to say.  Do you mean "This
> > won't reach the root node because the loop will break when the last leaf
> > is hit"?  I don't think that is accurate.. it will hit the root node but
> > not the end of the root node, right?  Anyways, the comment isn't clear
> > so please have a look.
> > 
> > > +				mas_ascend(mas);
> > > +				mas_ascend(new_mas);
> > > +			} while (mas->offset == mas_data_end(mas));
> > > +
> > > +			mas->offset++;
> > > +			new_mas->offset++;
> > > +		}
> > > +
> > > +		mas_descend(mas);
> > > +		parent = mte_to_node(new_mas->node);
> > > +		mas_descend(new_mas);
> > > +		mas->offset = 0;
> > > +		new_mas->offset = 0;
> > > +	}
> > > +}
> > > +
> > > +/**
> > > + * __mt_dup(): Duplicate a maple tree
> > > + * @mt: The source maple tree
> > > + * @new: The new maple tree
> > > + * @gfp: The GFP_FLAGS to use for allocations
> > > + *
> > > + * This function duplicates a maple tree using a faster method than traversing
> > > + * the source tree and inserting entries into the new tree one by one.
> > 
> > Can you make this comment more about what your code does instead of the
> > "one by one" description?
> > 
> > > + * The user needs to ensure that the attributes of the source tree and the new
> > > + * tree are the same, and the new tree needs to be an empty tree, otherwise
> > > + * -EINVAL will be returned.
> > > + * Note that the user needs to manually lock the source tree and the new tree.
> > > + *
> > > + * Return: 0 on success, -ENOMEM if memory could not be allocated, -EINVAL If
> > > + * the attributes of the two trees are different or the new tree is not an empty
> > > + * tree.
> > > + */
> > > +int __mt_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp)
> > > +{
> > > +	int ret = 0;
> > > +	MA_STATE(mas, mt, 0, 0);
> > > +	MA_STATE(new_mas, new, 0, 0);
> > > +
> > > +	mas_dup_build(&mas, &new_mas, gfp);
> > > +
> > > +	if (unlikely(mas_is_err(&mas))) {
> > > +		ret = xa_err(mas.node);
> > > +		if (ret == -ENOMEM)
> > > +			mas_dup_free(&new_mas);
> > > +	}
> > > +
> > > +	return ret;
> > > +}
> > > +EXPORT_SYMBOL(__mt_dup);
> > > +
> > > +/**
> > > + * mtree_dup(): Duplicate a maple tree
> > > + * @mt: The source maple tree
> > > + * @new: The new maple tree
> > > + * @gfp: The GFP_FLAGS to use for allocations
> > > + *
> > > + * This function duplicates a maple tree using a faster method than traversing
> > > + * the source tree and inserting entries into the new tree one by one.
> > 
> > Again, it's more interesting to state it uses the DFS preorder copy.
> > 
> > It is also worth mentioning the superior allocation behaviour since that
> > is a desirable trait for many.  In fact, you should add the allocation
> > behaviour in your cover letter.
> > 
> > > + * The user needs to ensure that the attributes of the source tree and the new
> > > + * tree are the same, and the new tree needs to be an empty tree, otherwise
> > > + * -EINVAL will be returned.
> > > + *
> > > + * Return: 0 on success, -ENOMEM if memory could not be allocated, -EINVAL If
> > > + * the attributes of the two trees are different or the new tree is not an empty
> > > + * tree.
> > > + */
> > > +int mtree_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp)
> > > +{
> > > +	int ret = 0;
> > > +	MA_STATE(mas, mt, 0, 0);
> > > +	MA_STATE(new_mas, new, 0, 0);
> > > +
> > > +	mas_lock(&new_mas);
> > > +	mas_lock(&mas);
> > > +
> > > +	mas_dup_build(&mas, &new_mas, gfp);
> > > +	mas_unlock(&mas);
> > > +
> > > +	if (unlikely(mas_is_err(&mas))) {
> > > +		ret = xa_err(mas.node);
> > > +		if (ret == -ENOMEM)
> > > +			mas_dup_free(&new_mas);
> > > +	}
> > > +
> > > +	mas_unlock(&new_mas);
> > > +
> > > +	return ret;
> > > +}
> > > +EXPORT_SYMBOL(mtree_dup);
> > > +
> > >   /**
> > >    * __mt_destroy() - Walk and free all nodes of a locked maple tree.
> > >    * @mt: The maple tree
> > > -- 
> > > 2.20.1
> > > 

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