[074/192] mm, swap: remove unnecessary smp_rmb() in swap_type_to_swap_info()

Commit Message

Andrew Morton June 29, 2021, 2:37 a.m. UTC

From: Huang Ying <[e-mail]>
Subject: mm, swap: remove unnecessary smp_rmb() in swap_type_to_swap_info()

Before commit c10d38cc8d3e ("mm, swap: bounds check swap_info array
accesses to avoid NULL derefs"), the typical code to reference the
swap_info[] is as follows,

  type = swp_type(swp_entry);
  if (type >= nr_swapfiles)
          /* handle invalid swp_entry */;
  p = swap_info[type];
  /* access fields of *p.  OOPS! p may be NULL! */

Because the ordering isn't guaranteed, it's possible that swap_info[type]
is read before "nr_swapfiles".  And that may result in NULL pointer
dereference.

So after commit c10d38cc8d3e, the code becomes,

  struct swap_info_struct *swap_type_to_swap_info(int type)
  {
	  if (type >= READ_ONCE(nr_swapfiles))
		  return NULL;
	  smp_rmb();
	  return READ_ONCE(swap_info[type]);
  }

  /* users */
  type = swp_type(swp_entry);
  p = swap_type_to_swap_info(type);
  if (!p)
	  /* handle invalid swp_entry */;
  /* dereference p */

Where the value of swap_info[type] (that is, "p") is checked to be
non-zero before being dereferenced.  So, the NULL deferencing becomes
impossible even if "nr_swapfiles" is read after swap_info[type]. 
Therefore, the "smp_rmb()" becomes unnecessary.

And, we don't even need to read "nr_swapfiles" here.  Because the non-zero
checking for "p" is sufficient.  We just need to make sure we will not
access out of the boundary of the array.  With the change, nr_swapfiles
will only be accessed with swap_lock held, except in
swapcache_free_entries().  Where the absolute correctness of the value
isn't needed, as described in the comments.

We still need to guarantee swap_info[type] is read before being
dereferenced.  That can be satisfied via the data dependency ordering
enforced by READ_ONCE(swap_info[type]).  This needs to be paired with
proper write barriers.  So smp_store_release() is used in
alloc_swap_info() to guarantee the fields of *swap_info[type] is
initialized before swap_info[type] itself being written.  Note that the
fields of *swap_info[type] is initialized to be 0 via kvzalloc() firstly. 
The assignment and deferencing of swap_info[type] is like
rcu_assign_pointer() and rcu_dereference().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: "Huang, Ying" <[e-mail]>
Cc: Daniel Jordan <[e-mail]>
Cc: Dan Carpenter <[e-mail]>
Cc: Andrea Parri <[e-mail]>
Cc: Peter Zijlstra (Intel) <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Omar Sandoval <[e-mail]>
Cc: Paul McKenney <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Miaohe Lin <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/swapfile.c |   15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

Patch

--- a/mm/swapfile.c~mm-swap-remove-unnecessary-smp_rmb-in-swap_type_to_swap_info
+++ a/mm/swapfile.c
@@ -100,11 +100,10 @@  atomic_t nr_rotate_swap = ATOMIC_INIT(0)
 
 static struct swap_info_struct *swap_type_to_swap_info(int type)
 {
-	if (type >= READ_ONCE(nr_swapfiles))
+	if (type >= MAX_SWAPFILES)
 		return NULL;
 
-	smp_rmb();	/* Pairs with smp_wmb in alloc_swap_info. */
-	return READ_ONCE(swap_info[type]);
+	return READ_ONCE(swap_info[type]); /* rcu_dereference() */
 }
 
 static inline unsigned char swap_count(unsigned char ent)
@@ -2863,14 +2862,12 @@  static struct swap_info_struct *alloc_sw
 	}
 	if (type >= nr_swapfiles) {
 		p->type = type;
-		WRITE_ONCE(swap_info[type], p);
 		/*
-		 * Write swap_info[type] before nr_swapfiles, in case a
-		 * racing procfs swap_start() or swap_next() is reading them.
-		 * (We never shrink nr_swapfiles, we never free this entry.)
+		 * Publish the swap_info_struct after initializing it.
+		 * Note that kvzalloc() above zeroes all its fields.
 		 */
-		smp_wmb();
-		WRITE_ONCE(nr_swapfiles, nr_swapfiles + 1);
+		smp_store_release(&swap_info[type], p); /* rcu_assign_pointer() */
+		nr_swapfiles++;
 	} else {
 		defer = p;
 		p = swap_info[type];