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Subject[PATCH 1/12] radix_tree: exceptional entries and indices
The radix_tree is used by several subsystems for different purposes.
A major use is to store the struct page pointers of a file's pagecache
for memory management. But what if mm wanted to store something other
than page pointers there too?

The low bit of a radix_tree entry is already used to denote an indirect
pointer, for internal use, and the unlikely radix_tree_deref_retry() case.
Define the next bit as denoting an exceptional entry, and supply inline
functions radix_tree_exception() to return non-0 in either unlikely case,
and radix_tree_exceptional_entry() to return non-0 in the second case.

If a subsystem already uses radix_tree with that bit set, no problem:
it does not affect internal workings at all, but is defined for the
convenience of those storing well-aligned pointers in the radix_tree.

The radix_tree_gang_lookups have an implicit assumption that the caller
can deduce the offset of each entry returned e.g. by the page->index of
a struct page. But that may not be feasible for some kinds of item to
be stored there.

radix_tree_gang_lookup_slot() allow for an optional indices argument,
output array in which to return those offsets. The same could be added
to other radix_tree_gang_lookups, but for now keep it to the only one
for which we need it.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
include/linux/radix-tree.h | 36 ++++++++++++++++++++++++++++++++---
lib/radix-tree.c | 29 ++++++++++++++++++----------
mm/filemap.c | 4 +--
3 files changed, 54 insertions(+), 15 deletions(-)
--- linux.orig/include/linux/radix-tree.h 2011-06-13 13:26:07.566101333 -0700
+++ linux/include/linux/radix-tree.h 2011-06-13 13:26:44.426284119 -0700
@@ -39,7 +39,15 @@
* when it is shrunk, before we rcu free the node. See shrink code for
* details.
*/
-#define RADIX_TREE_INDIRECT_PTR 1
+#define RADIX_TREE_INDIRECT_PTR 1
+/*
+ * A common use of the radix tree is to store pointers to struct pages;
+ * but shmem/tmpfs needs also to store swap entries in the same tree:
+ * those are marked as exceptional entries to distinguish them.
+ * EXCEPTIONAL_ENTRY tests the bit, EXCEPTIONAL_SHIFT shifts content past it.
+ */
+#define RADIX_TREE_EXCEPTIONAL_ENTRY 2
+#define RADIX_TREE_EXCEPTIONAL_SHIFT 2

#define radix_tree_indirect_to_ptr(ptr) \
radix_tree_indirect_to_ptr((void __force *)(ptr))
@@ -174,6 +182,28 @@ static inline int radix_tree_deref_retry
}

/**
+ * radix_tree_exceptional_entry - radix_tree_deref_slot gave exceptional entry?
+ * @arg: value returned by radix_tree_deref_slot
+ * Returns: 0 if well-aligned pointer, non-0 if exceptional entry.
+ */
+static inline int radix_tree_exceptional_entry(void *arg)
+{
+ /* Not unlikely because radix_tree_exception often tested first */
+ return (unsigned long)arg & RADIX_TREE_EXCEPTIONAL_ENTRY;
+}
+
+/**
+ * radix_tree_exception - radix_tree_deref_slot returned either exception?
+ * @arg: value returned by radix_tree_deref_slot
+ * Returns: 0 if well-aligned pointer, non-0 if either kind of exception.
+ */
+static inline int radix_tree_exception(void *arg)
+{
+ return unlikely((unsigned long)arg &
+ (RADIX_TREE_INDIRECT_PTR | RADIX_TREE_EXCEPTIONAL_ENTRY));
+}
+
+/**
* radix_tree_replace_slot - replace item in a slot
* @pslot: pointer to slot, returned by radix_tree_lookup_slot
* @item: new item to store in the slot.
@@ -194,8 +224,8 @@ void *radix_tree_delete(struct radix_tre
unsigned int
radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
unsigned long first_index, unsigned int max_items);
-unsigned int
-radix_tree_gang_lookup_slot(struct radix_tree_root *root, void ***results,
+unsigned int radix_tree_gang_lookup_slot(struct radix_tree_root *root,
+ void ***results, unsigned long *indices,
unsigned long first_index, unsigned int max_items);
unsigned long radix_tree_next_hole(struct radix_tree_root *root,
unsigned long index, unsigned long max_scan);
--- linux.orig/lib/radix-tree.c 2011-06-13 13:26:07.566101333 -0700
+++ linux/lib/radix-tree.c 2011-06-13 13:26:44.426284119 -0700
@@ -823,8 +823,8 @@ unsigned long radix_tree_prev_hole(struc
EXPORT_SYMBOL(radix_tree_prev_hole);

static unsigned int
-__lookup(struct radix_tree_node *slot, void ***results, unsigned long index,
- unsigned int max_items, unsigned long *next_index)
+__lookup(struct radix_tree_node *slot, void ***results, unsigned long *indices,
+ unsigned long index, unsigned int max_items, unsigned long *next_index)
{
unsigned int nr_found = 0;
unsigned int shift, height;
@@ -857,12 +857,16 @@ __lookup(struct radix_tree_node *slot, v

/* Bottom level: grab some items */
for (i = index & RADIX_TREE_MAP_MASK; i < RADIX_TREE_MAP_SIZE; i++) {
- index++;
if (slot->slots[i]) {
- results[nr_found++] = &(slot->slots[i]);
- if (nr_found == max_items)
+ results[nr_found] = &(slot->slots[i]);
+ if (indices)
+ indices[nr_found] = index;
+ if (++nr_found == max_items) {
+ index++;
goto out;
+ }
}
+ index++;
}
out:
*next_index = index;
@@ -918,8 +922,8 @@ radix_tree_gang_lookup(struct radix_tree

if (cur_index > max_index)
break;
- slots_found = __lookup(node, (void ***)results + ret, cur_index,
- max_items - ret, &next_index);
+ slots_found = __lookup(node, (void ***)results + ret, NULL,
+ cur_index, max_items - ret, &next_index);
nr_found = 0;
for (i = 0; i < slots_found; i++) {
struct radix_tree_node *slot;
@@ -944,6 +948,7 @@ EXPORT_SYMBOL(radix_tree_gang_lookup);
* radix_tree_gang_lookup_slot - perform multiple slot lookup on radix tree
* @root: radix tree root
* @results: where the results of the lookup are placed
+ * @indices: where their indices should be placed (but usually NULL)
* @first_index: start the lookup from this key
* @max_items: place up to this many items at *results
*
@@ -958,7 +963,8 @@ EXPORT_SYMBOL(radix_tree_gang_lookup);
* protection, radix_tree_deref_slot may fail requiring a retry.
*/
unsigned int
-radix_tree_gang_lookup_slot(struct radix_tree_root *root, void ***results,
+radix_tree_gang_lookup_slot(struct radix_tree_root *root,
+ void ***results, unsigned long *indices,
unsigned long first_index, unsigned int max_items)
{
unsigned long max_index;
@@ -974,6 +980,8 @@ radix_tree_gang_lookup_slot(struct radix
if (first_index > 0)
return 0;
results[0] = (void **)&root->rnode;
+ if (indices)
+ indices[0] = 0;
return 1;
}
node = indirect_to_ptr(node);
@@ -987,8 +995,9 @@ radix_tree_gang_lookup_slot(struct radix

if (cur_index > max_index)
break;
- slots_found = __lookup(node, results + ret, cur_index,
- max_items - ret, &next_index);
+ slots_found = __lookup(node, results + ret,
+ indices ? indices + ret : NULL,
+ cur_index, max_items - ret, &next_index);
ret += slots_found;
if (next_index == 0)
break;
--- linux.orig/mm/filemap.c 2011-06-13 13:26:07.566101333 -0700
+++ linux/mm/filemap.c 2011-06-13 13:26:44.430284135 -0700
@@ -843,7 +843,7 @@ unsigned find_get_pages(struct address_s
rcu_read_lock();
restart:
nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
- (void ***)pages, start, nr_pages);
+ (void ***)pages, NULL, start, nr_pages);
ret = 0;
for (i = 0; i < nr_found; i++) {
struct page *page;
@@ -906,7 +906,7 @@ unsigned find_get_pages_contig(struct ad
rcu_read_lock();
restart:
nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
- (void ***)pages, index, nr_pages);
+ (void ***)pages, NULL, index, nr_pages);
ret = 0;
for (i = 0; i < nr_found; i++) {
struct page *page;

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