mm: drop oom code from exit_mmap

mm: drop oom code from exit_mmap

The primary reason to invoke the oom reaper from the exit_mmap path used
to be a prevention of an excessive oom killing if the oom victim exit
races with the oom reaper (see [1] for more details).  The invocation has
moved around since then because of the interaction with the munlock logic
but the underlying reason has remained the same (see [2]).

Munlock code is no longer a problem since [3] and there shouldn't be any
blocking operation before the memory is unmapped by exit_mmap so the oom
reaper invocation can be dropped.  The unmapping part can be done with the
non-exclusive mmap_sem and the exclusive one is only required when page
tables are freed.

Remove the oom_reaper from exit_mmap which will make the code easier to
read.  This is really unlikely to make any observable difference although
some microbenchmarks could benefit from one less branch that needs to be
evaluated even though it almost never is true.

[1] 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
[2] 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3")
[3] a213e5cf71cb ("mm/munlock: delete munlock_vma_pages_all(), allow oomreap")

[[email protected]: restore Suren's mmap_read_lock() optimization]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Suren Baghdasaryan <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Christian Brauner (Microsoft) <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>