ida: Convert to XArray

Use the XA_TRACK_FREE ability to track which entries have a free bit,
similarly to how it uses the radix tree's IDR_FREE tag.  This eliminates
the per-cpu ida_bitmap preload, and fixes the memory consumption
regression I introduced when making the IDR able to store any pointer.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

2 files changed, 203 insertions, 247 deletions

diff --git a/lib/idr.c b/lib/idr.c
index 9a366b5be2c2..3c20ad9b0463 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -6,8 +6,6 @@
 #include <linux/spinlock.h>
 #include <linux/xarray.h>
 
-DEFINE_PER_CPU(struct ida_bitmap *, ida_bitmap);
-
 /**
  * idr_alloc_u32() - Allocate an ID.
  * @idr: IDR handle.
@@ -320,6 +318,9 @@ EXPORT_SYMBOL(idr_replace);
  * free the individual IDs in it.  You can use ida_is_empty() to find
  * out whether the IDA has any IDs currently allocated.
  *
+ * The IDA handles its own locking.  It is safe to call any of the IDA
+ * functions without synchronisation in your code.
+ *
  * IDs are currently limited to the range [0-INT_MAX].  If this is an awkward
  * limitation, it should be quite straightforward to raise the maximum.
  */
@@ -327,151 +328,197 @@ EXPORT_SYMBOL(idr_replace);
 /*
  * Developer's notes:
  *
- * The IDA uses the functionality provided by the IDR & radix tree to store
- * bitmaps in each entry.  The IDR_FREE tag means there is at least one bit
- * free, unlike the IDR where it means at least one entry is free.
+ * The IDA uses the functionality provided by the XArray to store bitmaps in
+ * each entry.  The XA_FREE_MARK is only cleared when all bits in the bitmap
+ * have been set.
  *
- * I considered telling the radix tree that each slot is an order-10 node
- * and storing the bit numbers in the radix tree, but the radix tree can't
- * allow a single multiorder entry at index 0, which would significantly
- * increase memory consumption for the IDA.  So instead we divide the index
- * by the number of bits in the leaf bitmap before doing a radix tree lookup.
+ * I considered telling the XArray that each slot is an order-10 node
+ * and indexing by bit number, but the XArray can't allow a single multi-index
+ * entry in the head, which would significantly increase memory consumption
+ * for the IDA.  So instead we divide the index by the number of bits in the
+ * leaf bitmap before doing a radix tree lookup.
  *
  * As an optimisation, if there are only a few low bits set in any given
  * leaf, instead of allocating a 128-byte bitmap, we store the bits
- * directly in the entry.
- *
- * We allow the radix tree 'exceptional' count to get out of date.  Nothing
- * in the IDA nor the radix tree code checks it.  If it becomes important
- * to maintain an accurate exceptional count, switch the rcu_assign_pointer()
- * calls to radix_tree_iter_replace() which will correct the exceptional
- * count.
- *
- * The IDA always requires a lock to alloc/free.  If we add a 'test_bit'
+ * as a value entry.  Value entries never have the XA_FREE_MARK cleared
+ * because we can always convert them into a bitmap entry.
+ *
+ * It would be possible to optimise further; once we've run out of a
+ * single 128-byte bitmap, we currently switch to a 576-byte node, put
+ * the 128-byte bitmap in the first entry and then start allocating extra
+ * 128-byte entries.  We could instead use the 512 bytes of the node's
+ * data as a bitmap before moving to that scheme.  I do not believe this
+ * is a worthwhile optimisation; Rasmus Villemoes surveyed the current
+ * users of the IDA and almost none of them use more than 1024 entries.
+ * Those that do use more than the 8192 IDs that the 512 bytes would
+ * provide.
+ *
+ * The IDA always uses a lock to alloc/free.  If we add a 'test_bit'
  * equivalent, it will still need locking.  Going to RCU lookup would require
  * using RCU to free bitmaps, and that's not trivial without embedding an
  * RCU head in the bitmap, which adds a 2-pointer overhead to each 128-byte
  * bitmap, which is excessive.
  */
 
-#define IDA_MAX (0x80000000U / IDA_BITMAP_BITS - 1)
-
-static int ida_get_new_above(struct ida *ida, int start)
+/**
+ * ida_alloc_range() - Allocate an unused ID.
+ * @ida: IDA handle.
+ * @min: Lowest ID to allocate.
+ * @max: Highest ID to allocate.
+ * @gfp: Memory allocation flags.
+ *
+ * Allocate an ID between @min and @max, inclusive.  The allocated ID will
+ * not exceed %INT_MAX, even if @max is larger.
+ *
+ * Context: Any context.
+ * Return: The allocated ID, or %-ENOMEM if memory could not be allocated,
+ * or %-ENOSPC if there are no free IDs.
+ */
+int ida_alloc_range(struct ida *ida, unsigned int min, unsigned int max,
+                        gfp_t gfp)
 {
-        struct radix_tree_root *root = &ida->ida_rt;
-        void __rcu **slot;
-        struct radix_tree_iter iter;
-        struct ida_bitmap *bitmap;
-        unsigned long index;
-        unsigned bit;
-        int new;
-
-        index = start / IDA_BITMAP_BITS;
-        bit = start % IDA_BITMAP_BITS;
-
-        slot = radix_tree_iter_init(&iter, index);
-        for (;;) {
-                if (slot)
-                        slot = radix_tree_next_slot(slot, &iter,
-                                                RADIX_TREE_ITER_TAGGED);
-                if (!slot) {
-                        slot = idr_get_free(root, &iter, GFP_NOWAIT, IDA_MAX);
-                        if (IS_ERR(slot)) {
-                                if (slot == ERR_PTR(-ENOMEM))
-                                        return -EAGAIN;
-                                return PTR_ERR(slot);
+        XA_STATE(xas, &ida->xa, min / IDA_BITMAP_BITS);
+        unsigned bit = min % IDA_BITMAP_BITS;
+        unsigned long flags;
+        struct ida_bitmap *bitmap, *alloc = NULL;
+
+        if ((int)min < 0)
+                return -ENOSPC;
+
+        if ((int)max < 0)
+                max = INT_MAX;
+
+retry:
+        xas_lock_irqsave(&xas, flags);
+next:
+        bitmap = xas_find_marked(&xas, max / IDA_BITMAP_BITS, XA_FREE_MARK);
+        if (xas.xa_index > min / IDA_BITMAP_BITS)
+                bit = 0;
+        if (xas.xa_index * IDA_BITMAP_BITS + bit > max)
+                goto nospc;
+
+        if (xa_is_value(bitmap)) {
+                unsigned long tmp = xa_to_value(bitmap);
+
+                if (bit < BITS_PER_XA_VALUE) {
+                        bit = find_next_zero_bit(&tmp, BITS_PER_XA_VALUE, bit);
+                        if (xas.xa_index * IDA_BITMAP_BITS + bit > max)
+                                goto nospc;
+                        if (bit < BITS_PER_XA_VALUE) {
+                                tmp |= 1UL << bit;
+                                xas_store(&xas, xa_mk_value(tmp));
+                                goto out;
                         }
                 }
-                if (iter.index > index)
-                        bit = 0;
-                new = iter.index * IDA_BITMAP_BITS;
-                bitmap = rcu_dereference_raw(*slot);
-                if (xa_is_value(bitmap)) {
-                        unsigned long tmp = xa_to_value(bitmap);
-                        int vbit = find_next_zero_bit(&tmp, BITS_PER_XA_VALUE,
-                                                        bit);
-                        if (vbit < BITS_PER_XA_VALUE) {
-                                tmp |= 1UL << vbit;
-                                rcu_assign_pointer(*slot, xa_mk_value(tmp));
-                                return new + vbit;
-                        }
-                        bitmap = this_cpu_xchg(ida_bitmap, NULL);
-                        if (!bitmap)
-                                return -EAGAIN;
-                        bitmap->bitmap[0] = tmp;
-                        rcu_assign_pointer(*slot, bitmap);
+                bitmap = alloc;
+                if (!bitmap)
+                        bitmap = kzalloc(sizeof(*bitmap), GFP_NOWAIT);
+                if (!bitmap)
+                        goto alloc;
+                bitmap->bitmap[0] = tmp;
+                xas_store(&xas, bitmap);
+                if (xas_error(&xas)) {
+                        bitmap->bitmap[0] = 0;
+                        goto out;
                 }
+        }
 
-                if (bitmap) {
-                        bit = find_next_zero_bit(bitmap->bitmap,
-                                                        IDA_BITMAP_BITS, bit);
-                        new += bit;
-                        if (new < 0)
-                                return -ENOSPC;
-                        if (bit == IDA_BITMAP_BITS)
-                                continue;
+        if (bitmap) {
+                bit = find_next_zero_bit(bitmap->bitmap, IDA_BITMAP_BITS, bit);
+                if (xas.xa_index * IDA_BITMAP_BITS + bit > max)
+                        goto nospc;
+                if (bit == IDA_BITMAP_BITS)
+                        goto next;
 
-                        __set_bit(bit, bitmap->bitmap);
-                        if (bitmap_full(bitmap->bitmap, IDA_BITMAP_BITS))
-                                radix_tree_iter_tag_clear(root, &iter,
-                                                                IDR_FREE);
+                __set_bit(bit, bitmap->bitmap);
+                if (bitmap_full(bitmap->bitmap, IDA_BITMAP_BITS))
+                        xas_clear_mark(&xas, XA_FREE_MARK);
+        } else {
+                if (bit < BITS_PER_XA_VALUE) {
+                        bitmap = xa_mk_value(1UL << bit);
                 } else {
-                        new += bit;
-                        if (new < 0)
-                                return -ENOSPC;
-                        if (bit < BITS_PER_XA_VALUE) {
-                                bitmap = xa_mk_value(1UL << bit);
-                        } else {
-                                bitmap = this_cpu_xchg(ida_bitmap, NULL);
-                                if (!bitmap)
-                                        return -EAGAIN;
-                                __set_bit(bit, bitmap->bitmap);
-                        }
-                        radix_tree_iter_replace(root, &iter, slot, bitmap);
+                        bitmap = alloc;
+                        if (!bitmap)
+                                bitmap = kzalloc(sizeof(*bitmap), GFP_NOWAIT);
+                        if (!bitmap)
+                                goto alloc;
+                        __set_bit(bit, bitmap->bitmap);
                 }
-
-                return new;
+                xas_store(&xas, bitmap);
+        }
+out:
+        xas_unlock_irqrestore(&xas, flags);
+        if (xas_nomem(&xas, gfp)) {
+                xas.xa_index = min / IDA_BITMAP_BITS;
+                bit = min % IDA_BITMAP_BITS;
+                goto retry;
         }
+        if (bitmap != alloc)
+                kfree(alloc);
+        if (xas_error(&xas))
+                return xas_error(&xas);
+        return xas.xa_index * IDA_BITMAP_BITS + bit;
+alloc:
+        xas_unlock_irqrestore(&xas, flags);
+        alloc = kzalloc(sizeof(*bitmap), gfp);
+        if (!alloc)
+                return -ENOMEM;
+        xas_set(&xas, min / IDA_BITMAP_BITS);
+        bit = min % IDA_BITMAP_BITS;
+        goto retry;
+nospc:
+        xas_unlock_irqrestore(&xas, flags);
+        return -ENOSPC;
 }
+EXPORT_SYMBOL(ida_alloc_range);
 
-static void ida_remove(struct ida *ida, int id)
+/**
+ * ida_free() - Release an allocated ID.
+ * @ida: IDA handle.
+ * @id: Previously allocated ID.
+ *
+ * Context: Any context.
+ */
+void ida_free(struct ida *ida, unsigned int id)
 {
-        unsigned long index = id / IDA_BITMAP_BITS;
-        unsigned offset = id % IDA_BITMAP_BITS;
+        XA_STATE(xas, &ida->xa, id / IDA_BITMAP_BITS);
+        unsigned bit = id % IDA_BITMAP_BITS;
         struct ida_bitmap *bitmap;
-        unsigned long *btmp;
-        struct radix_tree_iter iter;
-        void __rcu **slot;
+        unsigned long flags;
 
-        slot = radix_tree_iter_lookup(&ida->ida_rt, &iter, index);
-        if (!slot)
-                goto err;
+        BUG_ON((int)id < 0);
+
+        xas_lock_irqsave(&xas, flags);
+        bitmap = xas_load(&xas);
 
-        bitmap = rcu_dereference_raw(*slot);
         if (xa_is_value(bitmap)) {
-                btmp = (unsigned long *)slot;
-                offset += 1; /* Intimate knowledge of the value encoding */
-                if (offset >= BITS_PER_LONG)
+                unsigned long v = xa_to_value(bitmap);
+                if (bit >= BITS_PER_XA_VALUE)
+                        goto err;
+                if (!(v & (1UL << bit)))
                         goto err;
+                v &= ~(1UL << bit);
+                if (!v)
+                        goto delete;
+                xas_store(&xas, xa_mk_value(v));
         } else {
-                btmp = bitmap->bitmap;
-        }
-        if (!test_bit(offset, btmp))
-                goto err;
-
-        __clear_bit(offset, btmp);
-        radix_tree_iter_tag_set(&ida->ida_rt, &iter, IDR_FREE);
-        if (xa_is_value(bitmap)) {
-                if (xa_to_value(rcu_dereference_raw(*slot)) == 0)
-                        radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
-        } else if (bitmap_empty(btmp, IDA_BITMAP_BITS)) {
-                kfree(bitmap);
-                radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
+                if (!test_bit(bit, bitmap->bitmap))
+                        goto err;
+                __clear_bit(bit, bitmap->bitmap);
+                xas_set_mark(&xas, XA_FREE_MARK);
+                if (bitmap_empty(bitmap->bitmap, IDA_BITMAP_BITS)) {
+                        kfree(bitmap);
+delete:
+                        xas_store(&xas, NULL);
+                }
         }
+        xas_unlock_irqrestore(&xas, flags);
         return;
  err:
+        xas_unlock_irqrestore(&xas, flags);
         WARN(1, "ida_free called for id=%d which is not allocated.\n", id);
 }
+EXPORT_SYMBOL(ida_free);
 
 /**
  * ida_destroy() - Free all IDs.
@@ -486,80 +533,60 @@ static void ida_remove(struct ida *ida, int id)
  */
 void ida_destroy(struct ida *ida)
 {
+        XA_STATE(xas, &ida->xa, 0);
+        struct ida_bitmap *bitmap;
         unsigned long flags;
-        struct radix_tree_iter iter;
-        void __rcu **slot;
 
-        xa_lock_irqsave(&ida->ida_rt, flags);
-        radix_tree_for_each_slot(slot, &ida->ida_rt, &iter, 0) {
-                struct ida_bitmap *bitmap = rcu_dereference_raw(*slot);
+        xas_lock_irqsave(&xas, flags);
+        xas_for_each(&xas, bitmap, ULONG_MAX) {
                 if (!xa_is_value(bitmap))
                         kfree(bitmap);
-                radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
+                xas_store(&xas, NULL);
         }
-        xa_unlock_irqrestore(&ida->ida_rt, flags);
+        xas_unlock_irqrestore(&xas, flags);
 }
 EXPORT_SYMBOL(ida_destroy);
 
-/**
- * ida_alloc_range() - Allocate an unused ID.
- * @ida: IDA handle.
- * @min: Lowest ID to allocate.
- * @max: Highest ID to allocate.
- * @gfp: Memory allocation flags.
- *
- * Allocate an ID between @min and @max, inclusive.  The allocated ID will
- * not exceed %INT_MAX, even if @max is larger.
- *
- * Context: Any context.
- * Return: The allocated ID, or %-ENOMEM if memory could not be allocated,
- * or %-ENOSPC if there are no free IDs.
- */
-int ida_alloc_range(struct ida *ida, unsigned int min, unsigned int max,
-                        gfp_t gfp)
-{
-        int id = 0;
-        unsigned long flags;
-
-        if ((int)min < 0)
-                return -ENOSPC;
+#ifndef __KERNEL__
+extern void xa_dump_index(unsigned long index, unsigned int shift);
+#define IDA_CHUNK_SHIFT         ilog2(IDA_BITMAP_BITS)
 
-        if ((int)max < 0)
-                max = INT_MAX;
-
-again:
-        xa_lock_irqsave(&ida->ida_rt, flags);
-        id = ida_get_new_above(ida, min);
-        if (id > (int)max) {
-                ida_remove(ida, id);
-                id = -ENOSPC;
-        }
-        xa_unlock_irqrestore(&ida->ida_rt, flags);
+static void ida_dump_entry(void *entry, unsigned long index)
+{
+        unsigned long i;
+
+        if (!entry)
+                return;
+
+        if (xa_is_node(entry)) {
+                struct xa_node *node = xa_to_node(entry);
+                unsigned int shift = node->shift + IDA_CHUNK_SHIFT +
+                        XA_CHUNK_SHIFT;
+
+                xa_dump_index(index * IDA_BITMAP_BITS, shift);
+                xa_dump_node(node);
+                for (i = 0; i < XA_CHUNK_SIZE; i++)
+                        ida_dump_entry(node->slots[i],
+                                        index | (i << node->shift));
+        } else if (xa_is_value(entry)) {
+                xa_dump_index(index * IDA_BITMAP_BITS, ilog2(BITS_PER_LONG));
+                pr_cont("value: data %lx [%px]\n", xa_to_value(entry), entry);
+        } else {
+                struct ida_bitmap *bitmap = entry;
 
-        if (unlikely(id == -EAGAIN)) {
-                if (!ida_pre_get(ida, gfp))
-                        return -ENOMEM;
-                goto again;
+                xa_dump_index(index * IDA_BITMAP_BITS, IDA_CHUNK_SHIFT);
+                pr_cont("bitmap: %p data", bitmap);
+                for (i = 0; i < IDA_BITMAP_LONGS; i++)
+                        pr_cont(" %lx", bitmap->bitmap[i]);
+                pr_cont("\n");
         }
-
-        return id;
 }
-EXPORT_SYMBOL(ida_alloc_range);
 
-/**
- * ida_free() - Release an allocated ID.
- * @ida: IDA handle.
- * @id: Previously allocated ID.
- *
- * Context: Any context.
- */
-void ida_free(struct ida *ida, unsigned int id)
+static void ida_dump(struct ida *ida)
 {
-        unsigned long flags;
-
-        BUG_ON((int)id < 0);
-        xa_lock_irqsave(&ida->ida_rt, flags);
-        ida_remove(ida, id);
-        xa_unlock_irqrestore(&ida->ida_rt, flags);
+        struct xarray *xa = &ida->xa;
+        pr_debug("ida: %p node %p free %d\n", ida, xa->xa_head,
+                                xa->xa_flags >> ROOT_TAG_SHIFT);
+        ida_dump_entry(xa->xa_head, 0);
 }
-EXPORT_SYMBOL(ida_free);
+#endif
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 3479d93c32b9..68702061464f 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -255,54 +255,6 @@ static unsigned long next_index(unsigned long index,
         return (index & ~node_maxindex(node)) + (offset << node->shift);
 }
 
-#ifndef __KERNEL__
-static void dump_ida_node(void *entry, unsigned long index)
-{
-        unsigned long i;
-
-        if (!entry)
-                return;
-
-        if (radix_tree_is_internal_node(entry)) {
-                struct radix_tree_node *node = entry_to_node(entry);
-
-                pr_debug("ida node: %p offset %d indices %lu-%lu parent %p free %lx shift %d count %d\n",
-                        node, node->offset, index * IDA_BITMAP_BITS,
-                        ((index | node_maxindex(node)) + 1) *
-                                IDA_BITMAP_BITS - 1,
-                        node->parent, node->tags[0][0], node->shift,
-                        node->count);
-                for (i = 0; i < RADIX_TREE_MAP_SIZE; i++)
-                        dump_ida_node(node->slots[i],
-                                        index | (i << node->shift));
-        } else if (xa_is_value(entry)) {
-                pr_debug("ida excp: %p offset %d indices %lu-%lu data %lx\n",
-                                entry, (int)(index & RADIX_TREE_MAP_MASK),
-                                index * IDA_BITMAP_BITS,
-                                index * IDA_BITMAP_BITS + BITS_PER_XA_VALUE,
-                                xa_to_value(entry));
-        } else {
-                struct ida_bitmap *bitmap = entry;
-
-                pr_debug("ida btmp: %p offset %d indices %lu-%lu data", bitmap,
-                                (int)(index & RADIX_TREE_MAP_MASK),
-                                index * IDA_BITMAP_BITS,
-                                (index + 1) * IDA_BITMAP_BITS - 1);
-                for (i = 0; i < IDA_BITMAP_LONGS; i++)
-                        pr_cont(" %lx", bitmap->bitmap[i]);
-                pr_cont("\n");
-        }
-}
-
-static void ida_dump(struct ida *ida)
-{
-        struct radix_tree_root *root = &ida->ida_rt;
-        pr_debug("ida: %p node %p free %d\n", ida, root->xa_head,
-                                root->xa_flags >> ROOT_TAG_SHIFT);
-        dump_ida_node(root->xa_head, 0);
-}
-#endif
-
 /*
  * This assumes that the caller has performed appropriate preallocation, and
  * that the caller has pinned this thread of control to the current CPU.
@@ -2039,27 +1991,6 @@ void idr_preload(gfp_t gfp_mask)
 }
 EXPORT_SYMBOL(idr_preload);
 
-int ida_pre_get(struct ida *ida, gfp_t gfp)
-{
-        /*
-         * The IDA API has no preload_end() equivalent.  Instead,
-         * ida_get_new() can return -EAGAIN, prompting the caller
-         * to return to the ida_pre_get() step.
-         */
-        if (!__radix_tree_preload(gfp, IDA_PRELOAD_SIZE))
-                preempt_enable();
-
-        if (!this_cpu_read(ida_bitmap)) {
-                struct ida_bitmap *bitmap = kzalloc(sizeof(*bitmap), gfp);
-                if (!bitmap)
-                        return 0;
-                if (this_cpu_cmpxchg(ida_bitmap, NULL, bitmap))
-                        kfree(bitmap);
-        }
-
-        return 1;
-}
-
 void __rcu **idr_get_free(struct radix_tree_root *root,
                               struct radix_tree_iter *iter, gfp_t gfp,
                               unsigned long max)
@@ -2201,8 +2132,6 @@ static int radix_tree_cpu_dead(unsigned int cpu)
                 kmem_cache_free(radix_tree_node_cachep, node);
                 rtp->nr--;
         }
-        kfree(per_cpu(ida_bitmap, cpu));
-        per_cpu(ida_bitmap, cpu) = NULL;
         return 0;
 }