As the code evolves, some code paths that have previously assigned
deferred_work_generated may cease being reached. This would leave the value
uninitialized. This change initializes the value for safety.
Adding guarded extents, which are regular extents surrounded by guard pages
(mprotected). To reduce syscalls, small guarded extents are cached as a
separate eset in ecache, and decay through the dirty / muzzy / retained pipeline
as usual.
This mallctl accepts an arena_config_t structure which
can be used to customize the behavior of the arena.
Right now it contains extent_hooks and a new option,
metadata_use_hooks, which controls whether the extent
hooks are also used for metadata allocation.
The medata_use_hooks option has two main use cases:
1. In heterogeneous memory systems, to avoid metadata
being placed on potentially slower memory.
2. Avoiding virtual memory from being leaked as a result
of metadata allocation failure originating in an extent hook.
This change allows every allocator conforming to PAI communicate that it
deferred some work for the future. Without it if a background thread goes into
indefinite sleep, there is no way to notify it about upcoming deferred work.
For now, this is just a stub containing the ecaches, with no surrounding code
changed. Eventually all the core allocator bits will be moved in, in the
subsequent stack of commits.
