Re: [PATCH v7 1/8] xarray: add xas_try_split() to split a multi-index entry.

[David Hildenbrand <david@redhat.com>]

On 11.02.25 16:50, Zi Yan wrote:
> It is a preparation patch for non-uniform folio split, which always split
> a folio into half iteratively, and minimal xarray entry split.
> 
> Currently, xas_split_alloc() and xas_split() always split all slots from a
> multi-index entry. They cost the same number of xa_node as the to-be-split
> slots. For example, to split an order-9 entry, which takes 2^(9-6)=8
> slots, assuming XA_CHUNK_SHIFT is 6 (!CONFIG_BASE_SMALL), 8 xa_node are
> needed. Instead xas_try_split() is intended to be used iteratively to split
> the order-9 entry into 2 order-8 entries, then split one order-8 entry,
> based on the given index, to 2 order-7 entries, ..., and split one order-1
> entry to 2 order-0 entries. When splitting the order-6 entry and a new
> xa_node is needed, xas_try_split() will try to allocate one if possible.
> As a result, xas_try_split() would only need one xa_node instead of 8.
> 
> When a new xa_node is needed during the split, xas_try_split() can try to
> allocate one but no more. -ENOMEM will be return if a node cannot be
> allocated. -EINVAL will be return if a sibling node is split or
> cascade split happens, where two or more new nodes are needed, and these
> are not supported by xas_try_split().
> 
> xas_split_alloc() and xas_split() split an order-9 to order-0:
> 
>           ---------------------------------
>           |   |   |   |   |   |   |   |   |
>           | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
>           |   |   |   |   |   |   |   |   |
>           ---------------------------------
>             |   |                   |   |
>       -------   ---               ---   -------
>       |           |     ...       |           |
>       V           V               V           V
> ----------- -----------     ----------- -----------
> | xa_node | | xa_node | ... | xa_node | | xa_node |
> ----------- -----------     ----------- -----------
> 
> xas_try_split() splits an order-9 to order-0:
>     ---------------------------------
>     |   |   |   |   |   |   |   |   |
>     | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
>     |   |   |   |   |   |   |   |   |
>     ---------------------------------
>       |
>       |
>       V
> -----------
> | xa_node |
> -----------
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>   Documentation/core-api/xarray.rst |  14 ++-
>   include/linux/xarray.h            |   7 ++
>   lib/test_xarray.c                 |  47 +++++++++++
>   lib/xarray.c                      | 136 ++++++++++++++++++++++++++----
>   tools/testing/radix-tree/Makefile |   1 +
>   5 files changed, 188 insertions(+), 17 deletions(-)
> 
> diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
> index f6a3eef4fe7f..c6c91cbd0c3c 100644
> --- a/Documentation/core-api/xarray.rst
> +++ b/Documentation/core-api/xarray.rst
> @@ -489,7 +489,19 @@ Storing ``NULL`` into any index of a multi-index entry will set the
>   entry at every index to ``NULL`` and dissolve the tie.  A multi-index
>   entry can be split into entries occupying smaller ranges by calling
>   xas_split_alloc() without the xa_lock held, followed by taking the lock
> -and calling xas_split().
> +and calling xas_split() or calling xas_try_split() with xa_lock. The
> +difference between xas_split_alloc()+xas_split() and xas_try_alloc() is
> +that xas_split_alloc() + xas_split() split the entry from the original
> +order to the new order in one shot uniformly, whereas xas_try_split()
> +iteratively splits the entry containing the index non-uniformly.
> +For example, to split an order-9 entry, which takes 2^(9-6)=8 slots,
> +assuming ``XA_CHUNK_SHIFT`` is 6, xas_split_alloc() + xas_split() need
> +8 xa_node. xas_try_split() splits the order-9 entry into
> +2 order-8 entries, then split one order-8 entry, based on the given index,
> +to 2 order-7 entries, ..., and split one order-1 entry to 2 order-0 entries.
> +When splitting the order-6 entry and a new xa_node is needed, xas_try_split()
> +will try to allocate one if possible. As a result, xas_try_split() would only
> +need 1 xa_node instead of 8.
>   
>   Functions and structures
>   ========================
> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
> index 0b618ec04115..9eb8c7425090 100644
> --- a/include/linux/xarray.h
> +++ b/include/linux/xarray.h
> @@ -1555,6 +1555,8 @@ int xa_get_order(struct xarray *, unsigned long index);
>   int xas_get_order(struct xa_state *xas);
>   void xas_split(struct xa_state *, void *entry, unsigned int order);
>   void xas_split_alloc(struct xa_state *, void *entry, unsigned int order, gfp_t);
> +void xas_try_split(struct xa_state *xas, void *entry, unsigned int order,
> +		gfp_t gfp);
>   #else
>   static inline int xa_get_order(struct xarray *xa, unsigned long index)
>   {
> @@ -1576,6 +1578,11 @@ static inline void xas_split_alloc(struct xa_state *xas, void *entry,
>   		unsigned int order, gfp_t gfp)
>   {
>   }
> +
> +static inline void xas_try_split(struct xa_state *xas, void *entry,
> +		unsigned int order, gfp_t gfp)
> +{
> +}
>   #endif
>   
>   /**
> diff --git a/lib/test_xarray.c b/lib/test_xarray.c
> index 6932a26f4927..598ca38a2f5b 100644
> --- a/lib/test_xarray.c
> +++ b/lib/test_xarray.c
> @@ -1857,6 +1857,49 @@ static void check_split_1(struct xarray *xa, unsigned long index,
>   	xa_destroy(xa);
>   }
>   
> +static void check_split_2(struct xarray *xa, unsigned long index,
> +				unsigned int order, unsigned int new_order)
> +{
> +	XA_STATE_ORDER(xas, xa, index, new_order);
> +	unsigned int i, found;
> +	void *entry;
> +
> +	xa_store_order(xa, index, order, xa, GFP_KERNEL);
> +	xa_set_mark(xa, index, XA_MARK_1);
> +
> +	xas_lock(&xas);
> +	xas_try_halve(&xas, xa, order, GFP_KERNEL);
> +	if (((new_order / XA_CHUNK_SHIFT) < (order / XA_CHUNK_SHIFT)) &&
> +	    new_order < order - 1) {
> +		XA_BUG_ON(xa, !xas_error(&xas) || xas_error(&xas) != -EINVAL);
> +		xas_unlock(&xas);
> +		goto out;
> +	}
> +	for (i = 0; i < (1 << order); i += (1 << new_order))
> +		__xa_store(xa, index + i, xa_mk_index(index + i), 0);
> +	xas_unlock(&xas);
> +
> +	for (i = 0; i < (1 << order); i++) {
> +		unsigned int val = index + (i & ~((1 << new_order) - 1));
> +		XA_BUG_ON(xa, xa_load(xa, index + i) != xa_mk_index(val));
> +	}
> +
> +	xa_set_mark(xa, index, XA_MARK_0);
> +	XA_BUG_ON(xa, !xa_get_mark(xa, index, XA_MARK_0));
> +
> +	xas_set_order(&xas, index, 0);
> +	found = 0;
> +	rcu_read_lock();
> +	xas_for_each_marked(&xas, entry, ULONG_MAX, XA_MARK_1) {
> +		found++;
> +		XA_BUG_ON(xa, xa_is_internal(entry));
> +	}
> +	rcu_read_unlock();
> +	XA_BUG_ON(xa, found != 1 << (order - new_order));
> +out:
> +	xa_destroy(xa);
> +}
> +
>   static noinline void check_split(struct xarray *xa)
>   {
>   	unsigned int order, new_order;
> @@ -1868,6 +1911,10 @@ static noinline void check_split(struct xarray *xa)
>   			check_split_1(xa, 0, order, new_order);
>   			check_split_1(xa, 1UL << order, order, new_order);
>   			check_split_1(xa, 3UL << order, order, new_order);
> +
> +			check_split_2(xa, 0, order, new_order);
> +			check_split_2(xa, 1UL << order, order, new_order);
> +			check_split_2(xa, 3UL << order, order, new_order);
>   		}
>   	}
>   }
> diff --git a/lib/xarray.c b/lib/xarray.c
> index 116e9286c64e..c38beca77830 100644
> --- a/lib/xarray.c
> +++ b/lib/xarray.c
> @@ -1007,6 +1007,31 @@ static void node_set_marks(struct xa_node *node, unsigned int offset,
>   	}
>   }
>   
> +static struct xa_node *__xas_alloc_node_for_split(struct xa_state *xas,
> +		void *entry, gfp_t gfp)
> +{
> +	unsigned int i;
> +	void *sibling = NULL;
> +	struct xa_node *node;
> +	unsigned int mask = xas->xa_sibs;
> +
> +	node = kmem_cache_alloc_lru(radix_tree_node_cachep, xas->xa_lru, gfp);
> +	if (!node)
> +		return NULL;
> +	node->array = xas->xa;
> +	for (i = 0; i < XA_CHUNK_SIZE; i++) {
> +		if ((i & mask) == 0) {
> +			RCU_INIT_POINTER(node->slots[i], entry);
> +			sibling = xa_mk_sibling(i);
> +		} else {
> +			RCU_INIT_POINTER(node->slots[i], sibling);
> +		}
> +	}
> +	RCU_INIT_POINTER(node->parent, xas->xa_alloc);
> +
> +	return node;
> +}
> +
>   /**
>    * xas_split_alloc() - Allocate memory for splitting an entry.
>    * @xas: XArray operation state.
> @@ -1025,7 +1050,6 @@ void xas_split_alloc(struct xa_state *xas, void *entry, unsigned int order,
>   		gfp_t gfp)
>   {
>   	unsigned int sibs = (1 << (order % XA_CHUNK_SHIFT)) - 1;
> -	unsigned int mask = xas->xa_sibs;
>   
>   	/* XXX: no support for splitting really large entries yet */
>   	if (WARN_ON(xas->xa_shift + 2 * XA_CHUNK_SHIFT <= order))
> @@ -1034,23 +1058,9 @@ void xas_split_alloc(struct xa_state *xas, void *entry, unsigned int order,
>   		return;
>   
>   	do {
> -		unsigned int i;
> -		void *sibling = NULL;
> -		struct xa_node *node;
> -
> -		node = kmem_cache_alloc_lru(radix_tree_node_cachep, xas->xa_lru, gfp);
> +		struct xa_node *node = __xas_alloc_node_for_split(xas, entry, gfp);
>   		if (!node)
>   			goto nomem;
> -		node->array = xas->xa;
> -		for (i = 0; i < XA_CHUNK_SIZE; i++) {
> -			if ((i & mask) == 0) {
> -				RCU_INIT_POINTER(node->slots[i], entry);
> -				sibling = xa_mk_sibling(i);
> -			} else {
> -				RCU_INIT_POINTER(node->slots[i], sibling);
> -			}
> -		}
> -		RCU_INIT_POINTER(node->parent, xas->xa_alloc);
>   		xas->xa_alloc = node;
>   	} while (sibs-- > 0);
>   
> @@ -1122,6 +1132,100 @@ void xas_split(struct xa_state *xas, void *entry, unsigned int order)
>   	xas_update(xas, node);
>   }
>   EXPORT_SYMBOL_GPL(xas_split);
> +
> +/**
> + * xas_try_split() - Try to split a multi-index entry.
> + * @xas: XArray operation state.
> + * @entry: New entry to store in the array.
> + * @order: Current entry order.
> + * @gfp: Memory allocation flags.
> + *
> + * The size of the new entries is set in @xas.  The value in @entry is
> + * copied to all the replacement entries. If and only if one xa_node needs to
> + * be allocated, the function will use @gfp to get one. If more xa_node are
> + * needed, the function gives EINVAL error.
> + *
> + * Context: Any context.  The caller should hold the xa_lock.
> + */
> +void xas_try_split(struct xa_state *xas, void *entry, unsigned int order,
> +		gfp_t gfp)
> +{
> +	unsigned int sibs = (1 << (order % XA_CHUNK_SHIFT)) - 1;
> +	unsigned int offset, marks;
> +	struct xa_node *node;
> +	void *curr = xas_load(xas);
> +	int values = 0;
> +
> +	node = xas->xa_node;
> +	if (xas_top(node))
> +		return;
> +
> +	if (xas->xa->xa_flags & XA_FLAGS_ACCOUNT)
> +		gfp |= __GFP_ACCOUNT;
> +
> +	marks = node_get_marks(node, xas->xa_offset);
> +
> +	offset = xas->xa_offset + sibs;
> +	do {
> +		if (xas->xa_shift < node->shift) {
> +			struct xa_node *child = xas->xa_alloc;
> +			unsigned int expected_sibs =
> +				(1 << ((order - 1) % XA_CHUNK_SHIFT)) - 1;
> +
> +			/*
> +			 * No support for splitting sibling entries
> +			 * (horizontally) or cascade split (vertically), which
> +			 * requires two or more new xa_nodes.
> +			 * Since if one xa_node allocation fails,
> +			 * it is hard to free the prior allocations.
> +			 */
> +			if (sibs || xas->xa_sibs != expected_sibs) {
> +				xas_destroy(xas);
> +				xas_set_err(xas, -EINVAL);
> +				return;
> +			}
> +
> +			if (!child) {
> +				child = __xas_alloc_node_for_split(xas, entry,
> +						gfp);
> +				if (!child) {
> +					xas_destroy(xas);
> +					xas_set_err(xas, -ENOMEM);
> +					return;
> +				}
> +			}

No expert on this, just wondering ...

... what is the effect if we halfway-through fail the split? Is it okay 
to leave that "partially split" thing in place? Can callers deal with that?

-- 
Cheers,

David / dhildenb