[PATCH RFC] mm: relocate 'write_protect_seq' in struct mm_struct

[Feng Tang]

Before commit 57efa1fe5957 ("mm/gup: prevent gup_fast from
racing with COW during fork), on 64bits system, the hot member
rw_semaphore 'mmap_lock' of 'mm_struct' could be separated into
2 cachelines, that its member 'count' sits in one cacheline while
all other members in next cacheline, this naturally reduces some
cache bouncing, and with the commit, the 'mmap_lock' is pushed
into one cacheline, as shown in the pahole info:

 - before the commit

	spinlock_t         page_table_lock;      /*   116     4 */
	struct rw_semaphore mmap_lock;           /*   120    40 */
	/* --- cacheline 2 boundary (128 bytes) was 32 bytes ago --- */
	struct list_head   mmlist;               /*   160    16 */
	long unsigned int  hiwater_rss;          /*   176     8 */

 - after the commit

	spinlock_t         page_table_lock;      /*   124     4 */
	/* --- cacheline 2 boundary (128 bytes) --- */
	struct rw_semaphore mmap_lock;           /*   128    40 */
	struct list_head   mmlist;               /*   168    16 */
	long unsigned int  hiwater_rss;          /*   184     8 */

and it causes one 9.2% regression for 'mmap1' case of will-it-scale
benchmark[1], as in the case 'mmap_lock' is highly contented (occupies
90%+ cpu cycles).

Though relayouting a structure could be a double-edged sword, as it
helps some case, but may hurt other cases. So one solution is the
newly added 'seqcount_t' is 4 bytes long (when CONFIG_DEBUG_LOCK_ALLOC=n),
placing it into an existing 4 bytes hole in 'mm_struct' will not
affect most of other members's alignment, while restoring the
regression.

[1]. https://lore.kernel.org/lkml/20210525031636.GB7744@xsang-OptiPlex-9020/
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
 include/linux/mm_types.h | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5aacc1c..5b55f88 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -445,13 +445,6 @@ struct mm_struct {
 		 */
 		atomic_t has_pinned;
 
-		/**
-		 * @write_protect_seq: Locked when any thread is write
-		 * protecting pages mapped by this mm to enforce a later COW,
-		 * for instance during page table copying for fork().
-		 */
-		seqcount_t write_protect_seq;
-
 #ifdef CONFIG_MMU
 		atomic_long_t pgtables_bytes;	/* PTE page table pages */
 #endif
@@ -480,7 +473,15 @@ struct mm_struct {
 		unsigned long stack_vm;	   /* VM_STACK */
 		unsigned long def_flags;
 
+		/**
+		 * @write_protect_seq: Locked when any thread is write
+		 * protecting pages mapped by this mm to enforce a later COW,
+		 * for instance during page table copying for fork().
+		 */
+		seqcount_t write_protect_seq;
+
 		spinlock_t arg_lock; /* protect the below fields */
+
 		unsigned long start_code, end_code, start_data, end_data;
 		unsigned long start_brk, brk, start_stack;
 		unsigned long arg_start, arg_end, env_start, env_end;
-- 
2.7.4