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Commit 6328650b authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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radix_tree: exceptional entries and indices 



A patchset to extend tmpfs to MAX_LFS_FILESIZE by abandoning its
peculiar swap vector, instead keeping a file's swap entries in the same
radix tree as its struct page pointers: thus saving memory, and
simplifying its code and locking.

This patch:

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: default avatarHugh Dickins <hughd@google.com>
Acked-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 70d32719

include/linux/radix-tree.h

+33 −3
Original line number Diff line number Diff line
@@ -40,6 +40,14 @@ 
 * details.

 */

#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))
@@ -173,6 +181,28 @@ static inline int radix_tree_deref_retry(void *arg) 
	return unlikely((unsigned long)arg & RADIX_TREE_INDIRECT_PTR);

}



/**

 * 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
@@ -194,8 +224,8 @@ void *radix_tree_delete(struct radix_tree_root *, unsigned long); 
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);

lib/radix-tree.c

+19 −10
Original line number Diff line number Diff line
@@ -823,8 +823,8 @@ unsigned long radix_tree_prev_hole(struct radix_tree_root *root, 
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;
@@ -857,13 +857,17 @@ __lookup(struct radix_tree_node *slot, void ***results, unsigned long index, 


	/* 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;

	return nr_found;
@@ -918,8 +922,8 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results, 


		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_tree_root *root, void ***results, 
		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_tree_root *root, void ***results, 


		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;

mm/filemap.c

+2 −2
Original line number Diff line number Diff line
@@ -840,7 +840,7 @@ unsigned find_get_pages(struct address_space *mapping, pgoff_t start, 
	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;
@@ -903,7 +903,7 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index, 
	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;