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/47f6d39c-a075-50cb-1cfb-26dd957a48af@google.com
Fixes: b67bf49ce7aa ("mm/munlock: delete FOLL_MLOCK and FOLL_POPULATE")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Mike Galbraith <efault@gmx.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 271fbe8..f598a03 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1404,6 +1404,7 @@ long populate_vma_page_range(struct vm_area_struct *vma,
        struct mm_struct *mm = vma->vm_mm;
        unsigned long nr_pages = (end - start) / PAGE_SIZE;
        int gup_flags;
+       long ret;
 
        VM_BUG_ON(!PAGE_ALIGNED(start));
        VM_BUG_ON(!PAGE_ALIGNED(end));
@@ -1438,8 +1439,10 @@ long populate_vma_page_range(struct vm_area_struct *vma,
         * We made sure addr is within a VMA, so the following will
         * not result in a stack expansion that recurses back here.
         */
-       return __get_user_pages(mm, start, nr_pages, gup_flags,
+       ret = __get_user_pages(mm, start, nr_pages, gup_flags,
                                NULL, NULL, locked);
+       lru_add_drain();
+       return ret;
 }
 
 /*
@@ -1471,6 +1474,7 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
        struct mm_struct *mm = vma->vm_mm;
        unsigned long nr_pages = (end - start) / PAGE_SIZE;
        int gup_flags;
+       long ret;
 
        VM_BUG_ON(!PAGE_ALIGNED(start));
        VM_BUG_ON(!PAGE_ALIGNED(end));
@@ -1498,8 +1502,10 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
        if (check_vma_flags(vma, gup_flags))
                return -EINVAL;
 
-       return __get_user_pages(mm, start, nr_pages, gup_flags,
+       ret = __get_user_pages(mm, start, nr_pages, gup_flags,
                                NULL, NULL, locked);
+       lru_add_drain();
+       return ret;
 }
 
 /*