Very nice, nit inline.
For the series: Reviewed-by: William Kucharski <william.kucharski@xxxxxxxxxx>
> On Jun 29, 2020, at 9:20 AM, Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> wrote:
>
> In order to use multi-index entries for huge pages in the page cache,
> we need to be able to split a multi-index entry (eg if a file is
> truncated in the middle of a huge page entry). This version does not
> support splitting more than one level of the tree at a time. This is an
> acceptable limitation for the page cache as we do not expect to support
> order-12 pages in the near future.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> Documentation/core-api/xarray.rst | 16 ++--
> include/linux/xarray.h | 2 +
> lib/test_xarray.c | 41 ++++++++
> lib/xarray.c | 153 ++++++++++++++++++++++++++++--
> 4 files changed, 196 insertions(+), 16 deletions(-)
>
> diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
> index 640934b6f7b4..a137a0e6d068 100644
> --- a/Documentation/core-api/xarray.rst
> +++ b/Documentation/core-api/xarray.rst
> @@ -475,13 +475,15 @@ or iterations will move the index to the first index in the range.
> Each entry will only be returned once, no matter how many indices it
> occupies.
>
> -Using xas_next() or xas_prev() with a multi-index xa_state
> -is not supported. Using either of these functions on a multi-index entry
> -will reveal sibling entries; these should be skipped over by the caller.
> -
> -Storing ``NULL`` into any index of a multi-index entry will set the entry
> -at every index to ``NULL`` and dissolve the tie. Splitting a multi-index
> -entry into entries occupying smaller ranges is not yet supported.
> +Using xas_next() or xas_prev() with a multi-index xa_state is not
> +supported. Using either of these functions on a multi-index entry will
> +reveal sibling entries; these should be skipped over by the caller.
> +
> +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().
>
> Functions and structures
> ========================
> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
> index b4d70e7568b2..fd4364e17632 100644
> --- a/include/linux/xarray.h
> +++ b/include/linux/xarray.h
> @@ -1491,6 +1491,8 @@ static inline bool xas_retry(struct xa_state *xas, const void *entry)
>
> void *xas_load(struct xa_state *);
> void *xas_store(struct xa_state *, void *entry);
> +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_find(struct xa_state *, unsigned long max);
> void *xas_find_conflict(struct xa_state *);
>
> diff --git a/lib/test_xarray.c b/lib/test_xarray.c
> index acf5ccba58b0..282cce959aba 100644
> --- a/lib/test_xarray.c
> +++ b/lib/test_xarray.c
> @@ -1525,6 +1525,46 @@ static noinline void check_store_range(struct xarray *xa)
> }
> }
>
> +#ifdef CONFIG_XARRAY_MULTI
> +static void check_split_1(struct xarray *xa, unsigned long index,
> + unsigned int order)
> +{
> + XA_STATE_ORDER(xas, xa, index, order);
> + void *entry;
> + unsigned int i = 0;
> +
> + xa_store_order(xa, index, order, xa, GFP_KERNEL);
> +
> + xas_split_alloc(&xas, xa, 0, GFP_KERNEL);
> + xas_lock(&xas);
> + xas_split(&xas, xa, 0);
> + xas_unlock(&xas);
> +
> + xa_for_each(xa, index, entry) {
> + XA_BUG_ON(xa, entry != xa);
> + i++;
> + }
> + XA_BUG_ON(xa, i != 1 << order);
> +
> + xa_destroy(xa);
> +}
> +
> +static noinline void check_split(struct xarray *xa)
> +{
> + unsigned int order;
> +
> + XA_BUG_ON(xa, !xa_empty(xa));
> +
> + for (order = 1; order < 2 * XA_CHUNK_SHIFT; order++) {
It would be nice to have a comment here as to why you are checking
these particular size thresholds.
> + check_split_1(xa, 0, order);
> + check_split_1(xa, 1UL << order, order);
> + check_split_1(xa, 3UL << order, order);
> + }
> +}
> +#else
> +static void check_split(struct xarray *xa) { }
> +#endif
> +
> static void check_align_1(struct xarray *xa, char *name)
> {
> int i;
> @@ -1730,6 +1770,7 @@ static int xarray_checks(void)
> check_store_range(&array);
> check_store_iter(&array);
> check_align(&xa0);
> + check_split(&array);
>
> check_workingset(&array, 0);
> check_workingset(&array, 64);
> diff --git a/lib/xarray.c b/lib/xarray.c
> index e9e641d3c0c3..faca1ac95b0e 100644
> --- a/lib/xarray.c
> +++ b/lib/xarray.c
> @@ -266,13 +266,15 @@ static void xa_node_free(struct xa_node *node)
> */
> static void xas_destroy(struct xa_state *xas)
> {
> - struct xa_node *node = xas->xa_alloc;
> + struct xa_node *next, *node = xas->xa_alloc;
>
> - if (!node)
> - return;
> - XA_NODE_BUG_ON(node, !list_empty(&node->private_list));
> - kmem_cache_free(radix_tree_node_cachep, node);
> - xas->xa_alloc = NULL;
> + while (node) {
> + XA_NODE_BUG_ON(node, !list_empty(&node->private_list));
> + next = rcu_dereference_raw(node->parent);
> + /* XXX: need to free children */
> + kmem_cache_free(radix_tree_node_cachep, node);
> + xas->xa_alloc = node = next;
> + }
> }
>
> /**
> @@ -304,6 +306,7 @@ bool xas_nomem(struct xa_state *xas, gfp_t gfp)
> xas->xa_alloc = kmem_cache_alloc(radix_tree_node_cachep, gfp);
> if (!xas->xa_alloc)
> return false;
> + xas->xa_alloc->parent = NULL;
> XA_NODE_BUG_ON(xas->xa_alloc, !list_empty(&xas->xa_alloc->private_list));
> xas->xa_node = XAS_RESTART;
> return true;
> @@ -339,6 +342,7 @@ static bool __xas_nomem(struct xa_state *xas, gfp_t gfp)
> }
> if (!xas->xa_alloc)
> return false;
> + xas->xa_alloc->parent = NULL;
> XA_NODE_BUG_ON(xas->xa_alloc, !list_empty(&xas->xa_alloc->private_list));
> xas->xa_node = XAS_RESTART;
> return true;
> @@ -403,7 +407,7 @@ static unsigned long xas_size(const struct xa_state *xas)
> /*
> * Use this to calculate the maximum index that will need to be created
> * in order to add the entry described by @xas. Because we cannot store a
> - * multiple-index entry at index 0, the calculation is a little more complex
> + * multi-index entry at index 0, the calculation is a little more complex
> * than you might expect.
> */
> static unsigned long xas_max(struct xa_state *xas)
> @@ -946,6 +950,137 @@ void xas_init_marks(const struct xa_state *xas)
> }
> EXPORT_SYMBOL_GPL(xas_init_marks);
>
> +#ifdef CONFIG_XARRAY_MULTI
> +static unsigned int node_get_marks(struct xa_node *node, unsigned int offset)
> +{
> + unsigned int marks = 0;
> + xa_mark_t mark = XA_MARK_0;
> +
> + for (;;) {
> + if (node_get_mark(node, offset, mark))
> + marks |= 1 << (__force unsigned int)mark;
> + if (mark == XA_MARK_MAX)
> + break;
> + mark_inc(mark);
> + }
> +
> + return marks;
> +}
> +
> +static void node_set_marks(struct xa_node *node, unsigned int offset,
> + struct xa_node *child, unsigned int marks)
> +{
> + xa_mark_t mark = XA_MARK_0;
> +
> + for (;;) {
> + if (marks & (1 << (__force unsigned int)mark))
> + node_set_mark(node, offset, mark);
> + if (child)
> + node_mark_all(child, mark);
> + if (mark == XA_MARK_MAX)
> + break;
> + mark_inc(mark);
> + }
> +}
> +
> +void xas_split_alloc(struct xa_state *xas, void *entry, unsigned int order,
> + gfp_t gfp)
> +{
> + unsigned int sibs = xas->xa_sibs;
> + unsigned int mask = (1 << (order % XA_CHUNK_SHIFT)) - 1;
> +
> + /* XXX: no support for splitting really large entries yet */
> + if (WARN_ON(order + XA_CHUNK_SHIFT < xas->xa_shift))
> + goto nomem;
> + if (xas->xa_shift <= order)
> + return;
> +
> + do {
> + unsigned int i;
> + void *sibling;
> + struct xa_node *node;
> +
> + node = kmem_cache_alloc(radix_tree_node_cachep, 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(0);
> + } else {
> + RCU_INIT_POINTER(node->slots[i], sibling);
> + }
> + }
> + RCU_INIT_POINTER(node->parent, xas->xa_alloc);
> + xas->xa_alloc = node;
> + } while (sibs-- > 0);
> +
> + return;
> +nomem:
> + xas_destroy(xas);
> + xas_set_err(xas, -ENOMEM);
> +}
> +
> +/**
> + * xas_split() - Split a multi-index entry into smaller entries.
> + * @xas: XArray operation state.
> + * @entry: New entry to store in the array.
> + * @order: New entry order.
> + *
> + * The value in the entry is copied to all the replacement entries.
> + *
> + * Context: Any context. The caller should hold the xa_lock.
> + */
> +void xas_split(struct xa_state *xas, void *entry, unsigned int order)
> +{
> + 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;
> +
> + marks = node_get_marks(node, xas->xa_offset);
> +
> + offset = xas->xa_offset + xas->xa_sibs;
> + do {
> + if (order < node->shift) {
> + struct xa_node *child = xas->xa_alloc;
> +
> + xas->xa_alloc = rcu_dereference_raw(child->parent);
> + child->shift = node->shift - XA_CHUNK_SHIFT;
> + child->offset = offset;
> + child->count = XA_CHUNK_SIZE;
> + child->nr_values = xa_is_value(entry) ?
> + XA_CHUNK_SIZE : 0;
> + RCU_INIT_POINTER(child->parent, node);
> + node_set_marks(node, offset, child, marks);
> + rcu_assign_pointer(node->slots[offset],
> + xa_mk_node(child));
> + if (xa_is_value(curr))
> + values--;
> + } else {
> + unsigned int canon = offset -
> + (1 << (order % XA_CHUNK_SHIFT)) + 1;
> +
> + node_set_marks(node, canon, NULL, marks);
> + rcu_assign_pointer(node->slots[canon], entry);
> + while (offset > canon)
> + rcu_assign_pointer(node->slots[offset--],
> + xa_mk_sibling(canon));
> + values += (xa_is_value(entry) - xa_is_value(curr)) *
> + (1 << (order % XA_CHUNK_SHIFT));
> + }
> + } while (offset-- > xas->xa_offset);
> +
> + node->nr_values += values;
> +}
> +EXPORT_SYMBOL_GPL(xas_split);
> +#endif
> +
> /**
> * xas_pause() - Pause a walk to drop a lock.
> * @xas: XArray operation state.
> @@ -1407,7 +1542,7 @@ EXPORT_SYMBOL(__xa_store);
> * @gfp: Memory allocation flags.
> *
> * After this function returns, loads from this index will return @entry.
> - * Storing into an existing multislot entry updates the entry of every index.
> + * Storing into an existing multi-index entry updates the entry of every index.
> * The marks associated with @index are unaffected unless @entry is %NULL.
> *
> * Context: Any context. Takes and releases the xa_lock.
> @@ -1549,7 +1684,7 @@ static void xas_set_range(struct xa_state *xas, unsigned long first,
> *
> * After this function returns, loads from any index between @first and @last,
> * inclusive will return @entry.
> - * Storing into an existing multislot entry updates the entry of every index.
> + * Storing into an existing multi-index entry updates the entry of every index.
> * The marks associated with @index are unaffected unless @entry is %NULL.
> *
> * Context: Process context. Takes and releases the xa_lock. May sleep
> --
> 2.27.0
>
>
