mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts

Fix the races in maintaining compound_mapcount, subpages_mapcount and
subpage _mapcount by using PG_locked in the first tail of any compound
page for a bit_spin_lock() on such modifications; skipping the usual
atomic operations on those fields in this case.

Bring page_remove_file_rmap() and page_remove_anon_compound_rmap() back
into page_remove_rmap() itself.  Rearrange page_add_anon_rmap() and
page_add_file_rmap() and page_remove_rmap() to follow the same "if
(compound) {lock} else if (PageCompound) {lock} else {atomic}" pattern
(with a PageTransHuge in the compound test, like before, to avoid BUG_ONs
and optimize away that block when THP is not configured).  Move all the
stats updates outside, after the bit_spin_locked section, so that it is
sure to be a leaf lock.

Add page_dup_compound_rmap() to manage compound locking versus atomics in
sync with the rest.  In particular, hugetlb pages are still using the
atomics: to avoid unnecessary interference there, and because they never
have subpage mappings; but this exception can easily be changed. 
Conveniently, page_dup_compound_rmap() turns out to suit an anon THP's
__split_huge_pmd_locked() too.

bit_spin_lock() is not popular with PREEMPT_RT folks: but PREEMPT_RT
sensibly excludes TRANSPARENT_HUGEPAGE already, so its only exposure is to
the non-hugetlb non-THP pte-mapped compound pages (with large folios being
currently dependent on TRANSPARENT_HUGEPAGE).  There is never any scan of
subpages in this case; but we have chosen to use PageCompound tests rather
than PageTransCompound tests to gate the use of lock_compound_mapcounts(),
so that page_mapped() is correct on all compound pages, whether or not
TRANSPARENT_HUGEPAGE is enabled: could that be a problem for PREEMPT_RT,
when there is contention on the lock - under heavy concurrent forking for
example?  If so, then it can be turned into a sleeping lock (like
folio_lock()) when PREEMPT_RT.

A simple 100 X munmap(mmap(2GB, MAP_SHARED|MAP_POPULATE, tmpfs), 2GB) took
18 seconds on small pages, and used to take 1 second on huge pages, but
now takes 115 milliseconds on huge pages.  Mapping by pmds a second time
used to take 860ms and now takes 86ms; mapping by pmds after mapping by
ptes (when the scan is needed) used to take 870ms and now takes 495ms. 
Mapping huge pages by ptes is largely unaffected but variable: between 5%
faster and 5% slower in what I've recorded.  Contention on the lock is
likely to behave worse than contention on the atomics behaved.

Link: https://lkml.kernel.org/r/1b42bd1a-8223-e827-602f-d466c2db7d3c@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 9 insertions, 7 deletions

diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst
index a560e0c01b16..1e2a637cc607 100644
--- a/Documentation/mm/transhuge.rst
+++ b/Documentation/mm/transhuge.rst
@@ -117,13 +117,15 @@ pages:
   - ->_refcount in tail pages is always zero: get_page_unless_zero() never
     succeeds on tail pages.
 
-  - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
-    on relevant sub-page of the compound page.
-
-  - map/unmap of the whole compound page is accounted for in compound_mapcount
-    (stored in first tail page). For file huge pages, we also increment
-    ->_mapcount of all sub-pages in order to have race-free detection of
-    last unmap of subpages.
+  - map/unmap of PMD entry for the whole compound page increment/decrement
+    ->compound_mapcount, stored in the first tail page of the compound page.
+
+  - map/unmap of sub-pages with PTE entry increment/decrement ->_mapcount
+    on relevant sub-page of the compound page, and also increment/decrement
+    ->subpages_mapcount, stored in first tail page of the compound page.
+    In order to have race-free accounting of sub-pages mapped, changes to
+    sub-page ->_mapcount, ->subpages_mapcount and ->compound_mapcount are
+    are all locked by bit_spin_lock of PG_locked in the first tail ->flags.
 
 split_huge_page internally has to distribute the refcounts in the head
 page to the tail pages before clearing all PG_head/tail bits from the page