Merge tag 'xarray-5.5' of git://git.infradead.org/users/willy/linux-dax

Pull XArray fixes from Matthew Wilcox:
 "Primarily bugfixes, mostly around handling index wrap-around
  correctly.

  A couple of doc fixes and adding missing APIs.

  I had an oops live on stage at linux.conf.au this year, and it turned
  out to be a bug in xas_find() which I can't prove isn't triggerable in
  the current codebase. Then in looking for the bug, I spotted two more
  bugs.

  The bots have had a few days to chew on this with no problems
  reported, and it passes the test-suite (which now has more tests to
  make sure these problems don't come back)"

* tag 'xarray-5.5' of git://git.infradead.org/users/willy/linux-dax:
  XArray: Add xa_for_each_range
  XArray: Fix xas_find returning too many entries
  XArray: Fix xa_find_after with multi-index entries
  XArray: Fix infinite loop with entry at ULONG_MAX
  XArray: Add wrappers for nested spinlocks
  XArray: Improve documentation of search marks
  XArray: Fix xas_pause at ULONG_MAX

Documentation/core-api/xarray.rst

@@ -25,10 +25,6 @@ good performance with large indices.  If your index can be larger than
 ``ULONG_MAX`` then the XArray is not the data type for you.  The most
 important user of the XArray is the page cache.
 
-Each non-``NULL`` entry in the array has three bits associated with
-it called marks.  Each mark may be set or cleared independently of
-the others.  You can iterate over entries which are marked.
-
 Normal pointers may be stored in the XArray directly.  They must be 4-byte
 aligned, which is true for any pointer returned from kmalloc() and
 alloc_page().  It isn't true for arbitrary user-space pointers,
@@ -41,12 +37,11 @@ When you retrieve an entry from the XArray, you can check whether it is
 a value entry by calling xa_is_value(), and convert it back to
 an integer by calling xa_to_value().
 
-Some users want to store tagged pointers instead of using the marks
-described above.  They can call xa_tag_pointer() to create an
-entry with a tag, xa_untag_pointer() to turn a tagged entry
-back into an untagged pointer and xa_pointer_tag() to retrieve
-the tag of an entry.  Tagged pointers use the same bits that are used
-to distinguish value entries from normal pointers, so each user must
+Some users want to tag the pointers they store in the XArray.  You can
+call xa_tag_pointer() to create an entry with a tag, xa_untag_pointer()
+to turn a tagged entry back into an untagged pointer and xa_pointer_tag()
+to retrieve the tag of an entry.  Tagged pointers use the same bits that
+are used to distinguish value entries from normal pointers, so you must
 decide whether they want to store value entries or tagged pointers in
 any particular XArray.
 
@@ -56,10 +51,9 @@ conflict with value entries or internal entries.
 An unusual feature of the XArray is the ability to create entries which
 occupy a range of indices.  Once stored to, looking up any index in
 the range will return the same entry as looking up any other index in
-the range.  Setting a mark on one index will set it on all of them.
-Storing to any index will store to all of them.  Multi-index entries can
-be explicitly split into smaller entries, or storing ``NULL`` into any
-entry will cause the XArray to forget about the range.
+the range.  Storing to any index will store to all of them.  Multi-index
+entries can be explicitly split into smaller entries, or storing ``NULL``
+into any entry will cause the XArray to forget about the range.
 
 Normal API
 ==========
@@ -87,17 +81,11 @@ If you want to only store a new entry to an index if the current entry
 at that index is ``NULL``, you can use xa_insert() which
 returns ``-EBUSY`` if the entry is not empty.
 
-You can enquire whether a mark is set on an entry by using
-xa_get_mark().  If the entry is not ``NULL``, you can set a mark
-on it by using xa_set_mark() and remove the mark from an entry by
-calling xa_clear_mark().  You can ask whether any entry in the
-XArray has a particular mark set by calling xa_marked().
-
 You can copy entries out of the XArray into a plain array by calling
-xa_extract().  Or you can iterate over the present entries in
-the XArray by calling xa_for_each().  You may prefer to use
-xa_find() or xa_find_after() to move to the next present
-entry in the XArray.
+xa_extract().  Or you can iterate over the present entries in the XArray
+by calling xa_for_each(), xa_for_each_start() or xa_for_each_range().
+You may prefer to use xa_find() or xa_find_after() to move to the next
+present entry in the XArray.
 
 Calling xa_store_range() stores the same entry in a range
 of indices.  If you do this, some of the other operations will behave
@@ -124,6 +112,31 @@ xa_destroy().  If the XArray entries are pointers, you may wish
 to free the entries first.  You can do this by iterating over all present
 entries in the XArray using the xa_for_each() iterator.
 
+Search Marks
+------------
+
+Each entry in the array has three bits associated with it called marks.
+Each mark may be set or cleared independently of the others.  You can
+iterate over marked entries by using the xa_for_each_marked() iterator.
+
+You can enquire whether a mark is set on an entry by using
+xa_get_mark().  If the entry is not ``NULL``, you can set a mark on it
+by using xa_set_mark() and remove the mark from an entry by calling
+xa_clear_mark().  You can ask whether any entry in the XArray has a
+particular mark set by calling xa_marked().  Erasing an entry from the
+XArray causes all marks associated with that entry to be cleared.
+
+Setting or clearing a mark on any index of a multi-index entry will
+affect all indices covered by that entry.  Querying the mark on any
+index will return the same result.
+
+There is no way to iterate over entries which are not marked; the data
+structure does not allow this to be implemented efficiently.  There are
+not currently iterators to search for logical combinations of bits (eg
+iterate over all entries which have both ``XA_MARK_1`` and ``XA_MARK_2``
+set, or iterate over all entries which have ``XA_MARK_0`` or ``XA_MARK_2``
+set).  It would be possible to add these if a user arises.
+
 Allocating XArrays
 ------------------
 
@@ -180,6 +193,8 @@ No lock needed:
 Takes RCU read lock:
  * xa_load()
  * xa_for_each()
+ * xa_for_each_start()
+ * xa_for_each_range()
  * xa_find()
  * xa_find_after()
  * xa_extract()
@@ -419,10 +434,9 @@ you last processed.  If you have interrupts disabled while iterating,
 then it is good manners to pause the iteration and reenable interrupts
 every ``XA_CHECK_SCHED`` entries.
 
-The xas_get_mark(), xas_set_mark() and
-xas_clear_mark() functions require the xa_state cursor to have
-been moved to the appropriate location in the xarray; they will do
-nothing if you have called xas_pause() or xas_set()
+The xas_get_mark(), xas_set_mark() and xas_clear_mark() functions require
+the xa_state cursor to have been moved to the appropriate location in the
+XArray; they will do nothing if you have called xas_pause() or xas_set()
 immediately before.
 
 You can call xas_set_update() to have a callback function

include/linux/xarray.h

@@ -416,6 +416,36 @@ static inline bool xa_marked(const struct xarray *xa, xa_mark_t mark)
 	return xa->xa_flags & XA_FLAGS_MARK(mark);
 }
 
+/**
+ * xa_for_each_range() - Iterate over a portion of an XArray.
+ * @xa: XArray.
+ * @index: Index of @entry.
+ * @entry: Entry retrieved from array.
+ * @start: First index to retrieve from array.
+ * @last: Last index to retrieve from array.
+ *
+ * During the iteration, @entry will have the value of the entry stored
+ * in @xa at @index.  You may modify @index during the iteration if you
+ * want to skip or reprocess indices.  It is safe to modify the array
+ * during the iteration.  At the end of the iteration, @entry will be set
+ * to NULL and @index will have a value less than or equal to max.
+ *
+ * xa_for_each_range() is O(n.log(n)) while xas_for_each() is O(n).  You have
+ * to handle your own locking with xas_for_each(), and if you have to unlock
+ * after each iteration, it will also end up being O(n.log(n)).
+ * xa_for_each_range() will spin if it hits a retry entry; if you intend to
+ * see retry entries, you should use the xas_for_each() iterator instead.
+ * The xas_for_each() iterator will expand into more inline code than
+ * xa_for_each_range().
+ *
+ * Context: Any context.  Takes and releases the RCU lock.
+ */
+#define xa_for_each_range(xa, index, entry, start, last)		\
+	for (index = start,						\
+	     entry = xa_find(xa, &index, last, XA_PRESENT);		\
+	     entry;							\
+	     entry = xa_find_after(xa, &index, last, XA_PRESENT))
+
 /**
  * xa_for_each_start() - Iterate over a portion of an XArray.
  * @xa: XArray.
@@ -440,10 +470,7 @@ static inline bool xa_marked(const struct xarray *xa, xa_mark_t mark)
  * Context: Any context.  Takes and releases the RCU lock.
  */
 #define xa_for_each_start(xa, index, entry, start) \
-	for (index = start,						\
-	     entry = xa_find(xa, &index, ULONG_MAX, XA_PRESENT);	\
-	     entry;							\
-	     entry = xa_find_after(xa, &index, ULONG_MAX, XA_PRESENT))
+	xa_for_each_range(xa, index, entry, start, ULONG_MAX)
 
 /**
  * xa_for_each() - Iterate over present entries in an XArray.
@@ -508,6 +535,14 @@ static inline bool xa_marked(const struct xarray *xa, xa_mark_t mark)
 				spin_lock_irqsave(&(xa)->xa_lock, flags)
 #define xa_unlock_irqrestore(xa, flags) \
 				spin_unlock_irqrestore(&(xa)->xa_lock, flags)
+#define xa_lock_nested(xa, subclass) \
+				spin_lock_nested(&(xa)->xa_lock, subclass)
+#define xa_lock_bh_nested(xa, subclass) \
+				spin_lock_bh_nested(&(xa)->xa_lock, subclass)
+#define xa_lock_irq_nested(xa, subclass) \
+				spin_lock_irq_nested(&(xa)->xa_lock, subclass)
+#define xa_lock_irqsave_nested(xa, flags, subclass) \
+		spin_lock_irqsave_nested(&(xa)->xa_lock, flags, subclass)
 
 /*
  * Versions of the normal API which require the caller to hold the

lib/test_xarray.c

@@ -2,6 +2,7 @@
 /*
  * test_xarray.c: Test the XArray API
  * Copyright (c) 2017-2018 Microsoft Corporation
+ * Copyright (c) 2019-2020 Oracle
  * Author: Matthew Wilcox <willy@infradead.org>
  */
 
@@ -902,28 +903,34 @@ static noinline void check_store_iter(struct xarray *xa)
 	XA_BUG_ON(xa, !xa_empty(xa));
 }
 
-static noinline void check_multi_find(struct xarray *xa)
+static noinline void check_multi_find_1(struct xarray *xa, unsigned order)
 {
 #ifdef CONFIG_XARRAY_MULTI
+	unsigned long multi = 3 << order;
+	unsigned long next = 4 << order;
 	unsigned long index;
 
-	xa_store_order(xa, 12, 2, xa_mk_value(12), GFP_KERNEL);
-	XA_BUG_ON(xa, xa_store_index(xa, 16, GFP_KERNEL) != NULL);
+	xa_store_order(xa, multi, order, xa_mk_value(multi), GFP_KERNEL);
+	XA_BUG_ON(xa, xa_store_index(xa, next, GFP_KERNEL) != NULL);
+	XA_BUG_ON(xa, xa_store_index(xa, next + 1, GFP_KERNEL) != NULL);
 
 	index = 0;
 	XA_BUG_ON(xa, xa_find(xa, &index, ULONG_MAX, XA_PRESENT) !=
-			xa_mk_value(12));
-	XA_BUG_ON(xa, index != 12);
-	index = 13;
+			xa_mk_value(multi));
+	XA_BUG_ON(xa, index != multi);
+	index = multi + 1;
 	XA_BUG_ON(xa, xa_find(xa, &index, ULONG_MAX, XA_PRESENT) !=
-			xa_mk_value(12));
-	XA_BUG_ON(xa, (index < 12) || (index >= 16));
+			xa_mk_value(multi));
+	XA_BUG_ON(xa, (index < multi) || (index >= next));
 	XA_BUG_ON(xa, xa_find_after(xa, &index, ULONG_MAX, XA_PRESENT) !=
-			xa_mk_value(16));
-	XA_BUG_ON(xa, index != 16);
+			xa_mk_value(next));
+	XA_BUG_ON(xa, index != next);
+	XA_BUG_ON(xa, xa_find_after(xa, &index, next, XA_PRESENT) != NULL);
+	XA_BUG_ON(xa, index != next);
 
-	xa_erase_index(xa, 12);
-	xa_erase_index(xa, 16);
+	xa_erase_index(xa, multi);
+	xa_erase_index(xa, next);
+	xa_erase_index(xa, next + 1);
 	XA_BUG_ON(xa, !xa_empty(xa));
 #endif
 }
@@ -1046,12 +1053,33 @@ static noinline void check_find_3(struct xarray *xa)
 	xa_destroy(xa);
 }
 
+static noinline void check_find_4(struct xarray *xa)
+{
+	unsigned long index = 0;
+	void *entry;
+
+	xa_store_index(xa, ULONG_MAX, GFP_KERNEL);
+
+	entry = xa_find_after(xa, &index, ULONG_MAX, XA_PRESENT);
+	XA_BUG_ON(xa, entry != xa_mk_index(ULONG_MAX));
+
+	entry = xa_find_after(xa, &index, ULONG_MAX, XA_PRESENT);
+	XA_BUG_ON(xa, entry);
+
+	xa_erase_index(xa, ULONG_MAX);
+}
+
 static noinline void check_find(struct xarray *xa)
 {
+	unsigned i;
+
 	check_find_1(xa);
 	check_find_2(xa);
 	check_find_3(xa);
-	check_multi_find(xa);
+	check_find_4(xa);
+
+	for (i = 2; i < 10; i++)
+		check_multi_find_1(xa, i);
 	check_multi_find_2(xa);
 }
 
@@ -1132,6 +1160,27 @@ static noinline void check_move_tiny(struct xarray *xa)
 	XA_BUG_ON(xa, !xa_empty(xa));
 }
 
+static noinline void check_move_max(struct xarray *xa)
+{
+	XA_STATE(xas, xa, 0);
+
+	xa_store_index(xa, ULONG_MAX, GFP_KERNEL);
+	rcu_read_lock();
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != xa_mk_index(ULONG_MAX));
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != NULL);
+	rcu_read_unlock();
+
+	xas_set(&xas, 0);
+	rcu_read_lock();
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != xa_mk_index(ULONG_MAX));
+	xas_pause(&xas);
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != NULL);
+	rcu_read_unlock();
+
+	xa_erase_index(xa, ULONG_MAX);
+	XA_BUG_ON(xa, !xa_empty(xa));
+}
+
 static noinline void check_move_small(struct xarray *xa, unsigned long idx)
 {
 	XA_STATE(xas, xa, 0);
@@ -1240,6 +1289,7 @@ static noinline void check_move(struct xarray *xa)
 	xa_destroy(xa);
 
 	check_move_tiny(xa);
+	check_move_max(xa);
 
 	for (i = 0; i < 16; i++)
 		check_move_small(xa, 1UL << i);

lib/xarray.c

 // SPDX-License-Identifier: GPL-2.0+
 /*
  * XArray implementation
- * Copyright (c) 2017 Microsoft Corporation
+ * Copyright (c) 2017-2018 Microsoft Corporation
+ * Copyright (c) 2018-2020 Oracle
  * Author: Matthew Wilcox <willy@infradead.org>
  */
 
@@ -967,6 +968,7 @@ void xas_pause(struct xa_state *xas)
 	if (xas_invalid(xas))
 		return;
 
+	xas->xa_node = XAS_RESTART;
 	if (node) {
 		unsigned int offset = xas->xa_offset;
 		while (++offset < XA_CHUNK_SIZE) {
@@ -974,10 +976,11 @@ void xas_pause(struct xa_state *xas)
 				break;
 		}
 		xas->xa_index += (offset - xas->xa_offset) << node->shift;
+		if (xas->xa_index == 0)
+			xas->xa_node = XAS_BOUNDS;
 	} else {
 		xas->xa_index++;
 	}
-	xas->xa_node = XAS_RESTART;
 }
 EXPORT_SYMBOL_GPL(xas_pause);
 
@@ -1079,13 +1082,15 @@ void *xas_find(struct xa_state *xas, unsigned long max)
 {
 	void *entry;
 
-	if (xas_error(xas))
+	if (xas_error(xas) || xas->xa_node == XAS_BOUNDS)
 		return NULL;
+	if (xas->xa_index > max)
+		return set_bounds(xas);
 
 	if (!xas->xa_node) {
 		xas->xa_index = 1;
 		return set_bounds(xas);
-	} else if (xas_top(xas->xa_node)) {
+	} else if (xas->xa_node == XAS_RESTART) {
 		entry = xas_load(xas);
 		if (entry || xas_not_node(xas->xa_node))
 			return entry;
@@ -1150,6 +1155,8 @@ void *xas_find_marked(struct xa_state *xas, unsigned long max, xa_mark_t mark)
 
 	if (xas_error(xas))
 		return NULL;
+	if (xas->xa_index > max)
+		goto max;
 
 	if (!xas->xa_node) {
 		xas->xa_index = 1;
@@ -1824,6 +1831,17 @@ void *xa_find(struct xarray *xa, unsigned long *indexp,
 }
 EXPORT_SYMBOL(xa_find);
 
+static bool xas_sibling(struct xa_state *xas)
+{
+	struct xa_node *node = xas->xa_node;
+	unsigned long mask;
+
+	if (!node)
+		return false;
+	mask = (XA_CHUNK_SIZE << node->shift) - 1;
+	return (xas->xa_index & mask) > (xas->xa_offset << node->shift);
+}
+
 /**
  * xa_find_after() - Search the XArray for a present entry.
  * @xa: XArray.
@@ -1847,21 +1865,20 @@ void *xa_find_after(struct xarray *xa, unsigned long *indexp,
 	XA_STATE(xas, xa, *indexp + 1);
 	void *entry;
 
+	if (xas.xa_index == 0)
+		return NULL;
+
 	rcu_read_lock();
 	for (;;) {
 		if ((__force unsigned int)filter < XA_MAX_MARKS)
 			entry = xas_find_marked(&xas, max, filter);
 		else
 			entry = xas_find(&xas, max);
-		if (xas.xa_node == XAS_BOUNDS)
+
+		if (xas_invalid(&xas))
 			break;
-		if (xas.xa_shift) {
-			if (xas.xa_index & ((1UL << xas.xa_shift) - 1))
+		if (xas_sibling(&xas))
 			continue;
-		} else {
-			if (xas.xa_offset < (xas.xa_index & XA_CHUNK_MASK))
-				continue;
-		}
 		if (!xas_retry(&xas, entry))
 			break;
 	}