On Tue, Aug 28, 2018 at 03:27:27PM -0700, Matthew Wilcox wrote:

I find myself caught between two traditions.

On the one hand, the radix tree has been calling the page cache dirty &
writeback bits "tags" for over a decade.

On the other hand, using some of the bits _in a pointer_ as a tag has been
common practice since at least the 1960s.
https://en.wikipedia.org/wiki/Tagged_pointer and
https://en.wikipedia.org/wiki/31-bit

EROFS wants to use tagged pointers in the radix tree / xarray.  Right now,
they're building them by hand, which is predictably grotty-looking.
I think it's reasonable to provide this functionality as part of the
XArray API, _but_ it's confusing to have two different things called tags.

I've done my best to document my way around this, but if we want to rename
the things that the radix tree called tags to avoid the problem entirely,
now is the time to do it.  Anybody got a Good Idea?

I have two ideas now.

First, we could rename radix tree tags to xarray marks.  That is,

xa_mark_t
xa_set_mark()
xas_clear_mark()
xas_for_each_marked() { }
xa_marked()
etc

Second, we could call the tagged pointers typed pointers.  That is,

void *xa_mk_type(void *p, unsigned int type);
void *xa_to_ptr(void *entry);
int xa_ptr_type(void *entry);

Any better ideas, or violent revulsion to either of the above ideas?