mm: consolidate page table accounting

Currently, we account page tables separately for each page table level,
but that's redundant -- we only make use of total memory allocated to
page tables for oom_badness calculation.  We also provide the
information to userspace, but it has dubious value there too.

This patch switches page table accounting to single counter.

mm->pgtables_bytes is now used to account all page table levels.  We use
bytes, because page table size for different levels of page table tree
may be different.

The change has user-visible effect: we don't have VmPMD and VmPUD
reported in /proc/[pid]/status.  Not sure if anybody uses them.  (As
alternative, we can always report 0 kB for them.)

OOM-killer report is also slightly changed: we now report pgtables_bytes
instead of nr_ptes, nr_pmd, nr_puds.

Apart from reducing number of counters per-mm, the benefit is that we
now calculate oom_badness() more correctly for machines which have
different size of page tables depending on level or where page tables
are less than a page in size.

The only downside can be debuggability because we do not know which page
table level could leak.  But I do not remember many bugs that would be
caught by separate counters so I wouldn't lose sleep over this.

[akpm@linux-foundation.org: fix mm/huge_memory.c]
Link: http://lkml.kernel.org/r/20171006100651.44742-2-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
[kirill.shutemov@linux.intel.com: fix build]
  Link: http://lkml.kernel.org/r/20171016150113.ikfxy3e7zzfvsr4w@black.fi.intel.com
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 12 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 946922a30ede..006dc5899a1a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -817,9 +817,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
         init_rwsem(&mm->mmap_sem);
         INIT_LIST_HEAD(&mm->mmlist);
         mm->core_state = NULL;
-        mm_nr_ptes_init(mm);
-        mm_nr_pmds_init(mm);
-        mm_nr_puds_init(mm);
+        mm_pgtables_bytes_init(mm);
         mm->map_count = 0;
         mm->locked_vm = 0;
         mm->pinned_vm = 0;
@@ -873,15 +871,9 @@ static void check_mm(struct mm_struct *mm)
                                           "mm:%p idx:%d val:%ld\n", mm, i, x);
         }
 
-        if (mm_nr_ptes(mm))
-                pr_alert("BUG: non-zero nr_ptes on freeing mm: %ld\n",
-                                mm_nr_ptes(mm));
-        if (mm_nr_pmds(mm))
-                pr_alert("BUG: non-zero nr_pmds on freeing mm: %ld\n",
-                                mm_nr_pmds(mm));
-        if (mm_nr_puds(mm))
-                pr_alert("BUG: non-zero nr_puds on freeing mm: %ld\n",
-                                mm_nr_puds(mm));
+        if (mm_pgtables_bytes(mm))
+                pr_alert("BUG: non-zero pgtables_bytes on freeing mm: %ld\n",
+                                mm_pgtables_bytes(mm));
 
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
         VM_BUG_ON_MM(mm->pmd_huge_pte, mm);