Message-ID: <F5D2A2F4-34DC-4967-A149-A4D5578B5DB2@nvidia.com>
Date: Mon, 17 Feb 2025 17:05:30 -0500
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Ryan Roberts <ryan.roberts@....com>, Hugh Dickins <hughd@...gle.com>,
 Yang Shi <yang@...amperecomputing.com>, Miaohe Lin <linmiaohe@...wei.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Yu Zhao <yuzhao@...gle.com>,
 John Hubbard <jhubbard@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/8] xarray: add xas_try_split() to split a multi-index
 entry.

On 17 Feb 2025, at 16:44, David Hildenbrand wrote:

> 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@...dia.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?

Good question.

xas_try_split() imposes what kind of split it does and is usually used to
split from order N to order N-1:

1. when N is a multiplier of XA_CHUNK_SHIFT, a new xa_node is needed, so
either child (namely xas->xa_alloc) is not NULL, meaning someone called
xa_nomem() to allocate a xa_node before xas_try_split() or child is NULL
and an allocation is needed. If child is still NULL after the allocation,
meaning we are out of memory, no split is done;

2. when N is not, no new xa_node is needed, xas_try_split() just rewrites
existing slot values to perform the split (the code after the hunk above).
No fail will happen. For this split, since no new xa_node is needed,
the caller is actually allowed to split from N to a value smaller than
N-1 as long as N-1 is >= (N - N % XA_CHUNK_SHIFT).


Various checks make sure xas_try_split() only sees the two above situation:

a. "xas->xa_shift < node->shift" means the split crosses XA_CHUNK_SHIFT,
at least 1 new xa_node is needed; the else branch only handles the case
2 above;

b. for the then branch the "if (sibs || xas->xa_sibs != expected_sibs)"
check makes sure N is a multiplier of XA_CHUNK_SHIFT and the new order
has to be N-1. In "if (sibs || xas->xa_sibs != expected_sibs)",
"sibs != 0" means the from order N covers more than 1 slot, so more than 1
new xa_node is needed, "xas->xa_sibs != expected_sibs" makes sure
the new order is N-1 (you can see it from how expected_sibs is assigned).

Let me know if you have any other question.

--
Best Regards,
Yan, Zi

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