From 6b81141deb7358dbd64ce3e793cd1e3f5939f807 Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" Date: Fri, 8 Nov 2019 23:45:56 -0500 Subject: XArray: Improve documentation of search marks Move most of the mark-related documentation to its own section to make it easier to understand. Add clarification that you can't search for an unset mark, and you can't yet search for combinations of marks. Signed-off-by: Matthew Wilcox (Oracle) --- Documentation/core-api/xarray.rst | 60 +++++++++++++++++++++++---------------- 1 file changed, 36 insertions(+), 24 deletions(-) (limited to 'Documentation/core-api') diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst index fcedc5349ace..39b61ade7355 100644 --- a/Documentation/core-api/xarray.rst +++ b/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 ========== @@ -87,12 +81,6 @@ 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 @@ -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 ------------------ @@ -419,10 +432,9 @@ 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 -- cgit v1.2.3 From 00ed452c210a0bc1ff3ee79e1ce6b199f00a0638 Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" Date: Sun, 12 Jan 2020 15:54:10 -0500 Subject: XArray: Add xa_for_each_range This function supports iterating over a range of an array. Also add documentation links for xa_for_each_start(). Signed-off-by: Matthew Wilcox (Oracle) --- Documentation/core-api/xarray.rst | 10 ++++++---- include/linux/xarray.h | 37 ++++++++++++++++++++++++++++++++----- 2 files changed, 38 insertions(+), 9 deletions(-) (limited to 'Documentation/core-api') diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst index 39b61ade7355..640934b6f7b4 100644 --- a/Documentation/core-api/xarray.rst +++ b/Documentation/core-api/xarray.rst @@ -82,10 +82,10 @@ at that index is ``NULL``, you can use xa_insert() which returns ``-EBUSY`` if the entry is not empty. 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 @@ -193,6 +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() diff --git a/include/linux/xarray.h b/include/linux/xarray.h index 736afd3aa56f..f73e1775ded0 100644 --- a/include/linux/xarray.h +++ b/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. @@ -439,11 +469,8 @@ 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)) +#define xa_for_each_start(xa, index, entry, start) \ + xa_for_each_range(xa, index, entry, start, ULONG_MAX) /** * xa_for_each() - Iterate over present entries in an XArray. -- cgit v1.2.3