Re: [PATCH v8 1/8] xarray: add xas_try_split() to split a multi-index entry

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/2/26 23:00, Zi Yan wrote:
> On 26 Feb 2025, at 2:11, Baolin Wang wrote:
> 
>> Hi Zi,
>>
>> On 2025/2/19 07:50, Zi Yan wrote:
>>> 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>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Hugh Dickins <hughd@google.com>
>>> Cc: John Hubbard <jhubbard@nvidia.com>
>>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
>>> Cc: Miaohe Lin <linmiaohe@huawei.com>
>>> Cc: Matthew Wilcox <willy@infradead.org>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: Yang Shi <yang@os.amperecomputing.com>
>>> Cc: Yu Zhao <yuzhao@google.com>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> ---
>>>    Documentation/core-api/xarray.rst |  14 ++-
>>>    include/linux/xarray.h            |   7 ++
>>>    lib/test_xarray.c                 |  47 ++++++++++
>>>    lib/xarray.c                      | 138 ++++++++++++++++++++++++++----
>>>    tools/testing/radix-tree/Makefile |   1 +
>>>    5 files changed, 190 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
>>>     /**
>>
>> [snip]
>>
>>> diff --git a/lib/xarray.c b/lib/xarray.c
>>> index 116e9286c64e..b9a63d7fbd58 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 +1132,102 @@ 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)
>>
>> The xas_try_split() may sleep if ‘gfp’ flags permit while holding the xa_lock, which can cause issues. So can we add a check for the ‘gfp’ or only use GFP_NOWAIT?
> 
> You mean only allow gfp to be GFP_NOWAIT or GFP_ATOMIC?

Yes.