From: William Lee Irwin III <wli@holomorphy.com>

Micro-optimize sys_shmdt(). There are methods of exploiting knowledge
of the vma's being searched to restrict the search space. These are:

(1) shm mappings always start their lives at file offset 0, so only
	vma's above shmaddr need be considered. find_vma() can be used
	to seek to the proper position in mm->mmap in O(lg(n)) time.

(2) The search is for a vma which could be a fragment of a broken-up
	shm mapping, which would have been created starting at shmaddr
	with vm_pgoff 0 and then continued no further into userspace
	than shmaddr + size. So after having found an initial vma, find
	the size of the shm segment it maps to calculate an upper bound
	to the virtualspace that needs to be searched.

(3) mremap() would have caused the original checks to miss vma's mapping
	the shm segment if shmaddr were the original address at which
	the shm segments were attached. This does no better and no worse
	than the original code in that situation.

(4) If the chain of references in vma->vm_file->f_dentry->d_inode->i_size
	is not guaranteed by refcounting and/or the shm code then this is
	oopsable; AFAICT an inode is always allocated.