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@xxxxxxxxxx>
>> ---
>> 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
