FROMLIST: mm/oom_kill: allow process_mrelease to run under mmap_lock protection

With exit_mmap holding mmap_write_lock during free_pgtables call,
process_mrelease does not need to elevate mm->mm_users in order to
prevent exit_mmap from destrying pagetables while __oom_reap_task_mm
is walking the VMA tree. The change prevents process_mrelease from
calling the last mmput, which can lead to waiting for IO completion
in exit_aio.

Fixes: 337546e8 ("mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Link: https://lore.kernel.org/all/20211124235906.14437-2-surenb@google.com/



Bug: 130172058
Bug: 189803002
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I1e2728e0c477af9cc20e9e0b715ee67dee760618