The old implementation used the real lock_memory(). This is problematic
and does not work for a large number of reasons:
1) Various parts of the kernel assume memory is locked only very
temporarily, and will often wait on locked memory to become unlocked.
The transient nature of locks is further demonstrated by the fact that
lock_memory acquires references to structures, like the address space,
which are only released by unlock_memory
2) The VM has a hard assumption that all lock_memory calls will be
exactly balanced, and maintains internal "WiredRange" structures
on areas, etc. corresponding to the original lock_memory calls.
Maintaining separate data structures as this code did is a recipe
for even more problems when the structures are manipulated separately,
leading to confusing or incorrect behavior on unlocks.
3) Areas with locked memory cannot be deleted, nor can the pages which are
locked be removed from the areas/caches. This of course is most notable
when destroying teams which locked memory, but the problem also occurs
when just using delete_area, resize_area, mmap/munmap, etc.
Because of (2) and especially (3), adding support for mlock()-like semantics
to the existing memory locking system is just not a good option. A further
reason is that our lock_memory is much stricter than mlock(), which only
demands the pages in question must remain resident in RAM and cannot be
swapped out (or, it seems, otherwise written back to disk.)
Thus, this commit completely removes the old implementation (which
was seriously broken and did not actually automatically unlock memory
on team exit or area destruction at all, etc.) and instead adds a new
feature to VMAnonymousCache to block certain pages from being written out.
The syscall then just invokes this to do its work.
Fixes#17674. Related to #13651.
Change-Id: Id2745c51796bcf9a74ba5325fe686a95623cd521
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5147
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
c25f6f53b5