Eventually, we may fully break off the extent module; but not for some time. If
it's going to live on in a non-transitory state, it might as well have the nicer
name.
What we call an arena_ind is really the index associated with some particular
set of ehooks; the arena is just the user-visible portion of that. Making this
explicit, and reframing checks in terms of that, makes the code simpler and
cleaner, and helps us avoid passing the arena itself all throughout extent code.
This lets us put back an arena-specific assert.
Previously, it was really more like extents_alloc (it looks in an ecache for an
extent to reuse as its primary allocation pathway). Make that pathway more
explciitly like extents_alloc, and rename extent_alloc_wrapper_hard accordingly.
This will eventually completely wrap the eset, and handle concurrency,
allocation, and deallocation. For now, we only pull out the mutex from the
eset.
If there are custom extent hooks, pages_can_purge_lazy is not necessarily the
right guard. We could check ehooks_are_default too, but the case where
purge_lazy is unsupported is rare and getting rarer. Just checking the decay
interval captures most of the benefit.
We have to work to circumvent the safety checks in pre_reentrancy when going
down extent hook pathways. Instead, let's explicitly have checked and unchecked
guards.
When deferred initialization was added, initializing required copying
sizeof(extent_hooks_t) bytes after a pointer chase. Today, it's just a single
pointer loaded from the base_t. In subsequent diffs, we'll get rid of even that.
Explicitly define three setters:
- `prof_tctx_reset()`: set `prof_tctx` to `1U`, if we don't know in
advance whether the allocation is large or not;
- `prof_tctx_reset_sampled()`: set `prof_tctx` to `1U`, if we already
know in advance that the allocation is large;
- `prof_info_set()`: set a real `prof_tctx`, and also set other
profiling info e.g. the allocation time.
Code structure wise, the prof level is kept as a thin wrapper, the
large level only provides low level setter APIs, and the arena level
carries out the main logic.
