On Mon, Dec 11, 2017 at 03:10:22PM -0800, Randy Dunlap wrote: > > +A freshly-initialised XArray contains a ``NULL`` pointer at every index. > > +Each non-``NULL`` entry in the array has three bits associated with > > +it called tags. Each tag may be flipped on or off independently of > > +the others. You can search for entries with a given tag set. > > Only tags that are set, or search for entries with some tag(s) cleared? > Or is that like a mathematical set? hmm ... "Each tag may be set or cleared independently of the others. You can search for entries which have a particular tag set." Doesn't completely remove the ambiguity, but I can't think of how to phrase that better ... > > +Normal pointers may be stored in the XArray directly. They must be 4-byte > > +aligned, which is true for any pointer returned from :c:func:`kmalloc` and > > +:c:func:`alloc_page`. It isn't true for arbitrary user-space pointers, > > +nor for function pointers. You can store pointers to statically allocated > > +objects, as long as those objects have an alignment of at least 4. > > This (above) is due to the internal usage of low bits for flags? Sort of ... if bit 0 is set then we're storing an integer (see below), and if bit 0 is clear and bit 1 is set, then it's an internal entry. But I don't want the implementation details to leak into the user manual. >From the user's point of view, they can store a pointer to anything they allocated with kmalloc. If they want to store an arbitrary pointer, they're out of luck. > > +The XArray does not support storing :c:func:`IS_ERR` pointers; some > > +conflict with data values and others conflict with entries the XArray > > +uses for its own purposes. If you need to store special values which > > +cannot be confused with real kernel pointers, the values 4, 8, ... 4092 > > +are available. > > or if I know that they values are errno-range values, I can just shift them > left by 2 to store them and then shift them right by 2 to use them? Yes, the values -4 to -4092 also make good error signals. > oh, or use the following function? > > > +You can also store integers between 0 and ``LONG_MAX`` in the XArray. > > +You must first convert it into an entry using :c:func:`xa_mk_value`. > > +When you retrieve an entry from the XArray, you can check whether it is > > +a data value by calling :c:func:`xa_is_value`, and convert it back to > > +an integer by calling :c:func:`xa_to_value`. Yup, you could also store errors as integers, if you like. Your choice :-) > > +You can enquire whether a tag is set on an entry by using > > +:c:func:`xa_get_tag`. If the entry is not ``NULL``, you can set a tag > > +on it by using :c:func:`xa_set_tag` and remove the tag from an entry by > > +calling :c:func:`xa_clear_tag`. You can ask whether any entry in the > > an entry > > > +XArray has a particular tag set by calling :c:func:`xa_tagged`. > > or maybe I don't understand. Does xa_tagged() test one entry and return its > "tagged" result/status? or does it test (potentially) the entire array to search > for a particular tag value? It asks the question "Does any entry in the array have tag N set?" > > +If the xa_state is holding an %ENOMEM error, calling :c:func:`xas_nomem` > > +will attempt to allocate more memory using the specified gfp flags and > > +cache it in the xa_state for the next attempt. The idea is that you take > > +the xa_lock, attempt the operation and drop the lock. The operation > > +attempts to allocate memory while holding the lock, but it is more > > +likely to fail. Once you have dropped the lock, :c:func:`xas_nomem` > > +can try harder to allocate more memory. It will return ``true`` if it > > +is worth retrying the operation (ie that there was a memory error *and* > > usually i.e. > > > +more memory was allocated. If it has previously allocated memory, and > > allocated). Thanks! > > +If you need to move to a different index in the XArray, call > > +:c:func:`xas_set`. This reinitialises the cursor which will generally > > I would put a comma .... here.......................^ > but consult your $editor. :) I'll ask her, but I think you're right :-) > Nicely done. Thanks. Thanks for the review! I think we're still struggling a little to talk about tags in an unambiguous way, but apart from that it feels pretty good.
