[PATCH v2 0/3] mm: __access_remote_vm with per-VMA lock

[Rik van Riel <riel@surriel.com>]

Sometimes processes can get stuck with the mmap_lock held for
a long time. This slows down, and can even prevent system monitoring
tools from assessing and logging the situation, because they themselves
end up getting stuck on the mmap_lock.

However, with the introduction of per-VMA locks, we can improve the
reliability of system monitoring, and generally speed up __access_remote_vm
under mmap_loc contention, by adding a fast path that does not require
the process-wide mmap_lock.

This fast path is only compiled in and used when it is safe to do so,
meaning a kernel with per-VMA locks, RCU pgae table freeing, the VMA
is not hugetlbfs, iomap, pfnmap, etc...

v2:
 - simplify the code, which should be ok because these copies are < PAGE_SIZE
 - clean up the code
 - fix locking wrt tlb_remove_table_sync_one()
 - hopefully address all the other comments