In the current design, khugepaged needs to acquire mmap_sem before
scanning an mm.  But in some corner cases, khugepaged may scan a process
which is modifying its memory mapping, so khugepaged blocks in
uninterruptible state.  But the process might hold the mmap_sem for a
long time when modifying a huge memory space and it may trigger the
below khugepaged hung issue:

  INFO: task khugepaged:270 blocked for more than 120 seconds.
  Tainted: G E 4.9.65-006.ali3000.alios7.x86_64 #1
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  khugepaged D 0 270 2 0x00000000 
  ffff883f3deae4c0 0000000000000000 ffff883f610596c0 ffff883f7d359440
  ffff883f63818000 ffffc90019adfc78 ffffffff817079a5 d67e5aa8c1860a64
  0000000000000246 ffff883f7d359440 ffffc90019adfc88 ffff883f610596c0
  Call Trace:
    schedule+0x36/0x80
    rwsem_down_read_failed+0xf0/0x150
    call_rwsem_down_read_failed+0x18/0x30
    down_read+0x20/0x40
    khugepaged+0x476/0x11d0
    kthread+0xe6/0x100
    ret_from_fork+0x25/0x30

So it sounds pointless to just block khugepaged waiting for the
semaphore so replace down_read() with down_read_trylock() to move to
scan the next mm quickly instead of just blocking on the semaphore so
that other processes can get more chances to install THP.  Then
khugepaged can come back to scan the skipped mm when it has finished the
current round full_scan.

And it appears that the change can improve khugepaged efficiency a
little bit.

Below is the test result when running LTP on a 24 cores 4GB memory 2
nodes NUMA VM:

                                    pristine          w/ trylock
  full_scan                         197               187
  pages_collapsed                   21                26
  thp_fault_alloc                   40818             44466
  thp_fault_fallback                18413             16679
  thp_collapse_alloc                21                150
  thp_collapse_alloc_failed         14                16
  thp_file_alloc                    369               369

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: tweak comment]
[arnd@arndb.de: avoid uninitialized variable use]
  Link: http://lkml.kernel.org/r/20171215125129.2948634-1-arnd@arndb.de
Link: http://lkml.kernel.org/r/1513281203-54878-1-git-send-email-yang.s@alibaba-inc.comSigned-off-by: Yang Shi <yang.s@alibaba-inc.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>