mm/munlock: add lru_add_drain() to fix memcg_stat_test

Mike reports that LTP memcg_stat_test usually leads to

  memcg_stat_test 3 TINFO: Test unevictable with MAP_LOCKED
  memcg_stat_test 3 TINFO: Running memcg_process --mmap-lock1 -s 135168
  memcg_stat_test 3 TINFO: Warming up pid: 3460
  memcg_stat_test 3 TINFO: Process is still here after warm up: 3460
  memcg_stat_test 3 TFAIL: unevictable is 122880, 135168 expected

but may also lead to

  memcg_stat_test 4 TINFO: Test unevictable with mlock
  memcg_stat_test 4 TINFO: Running memcg_process --mmap-lock2 -s 135168
  memcg_stat_test 4 TINFO: Warming up pid: 4271
  memcg_stat_test 4 TINFO: Process is still here after warm up: 4271
  memcg_stat_test 4 TFAIL: unevictable is 122880, 135168 expected

or both.  A wee bit flaky.

follow_page_pte() used to have an lru_add_drain() per each page mlocked,
and the test came to rely on accurate stats.  The pagevec to be drained
is different now, but still covered by lru_add_drain(); and, never mind
the test, I believe it's in everyone's interest that a bulk faulting
interface like populate_vma_page_range() or faultin_vma_page_range()
should drain its local pagevecs at the end, to save others sometimes
needing the much more expensive lru_add_drain_all().

This does not absolutely guarantee exact stats - the mlocking task can
be migrated between CPUs as it proceeds - but it's good enough and the
tests pass.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: b67bf49ce7aa ("mm/munlock: delete FOLL_MLOCK and FOLL_POPULATE")
Signed-off-by: Hugh Dickins <[email protected]>
Reported-by: Mike Galbraith <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions