From a4b451143fa275a31f17a93adac3b8dbb3d20ca2 Mon Sep 17 00:00:00 2001 From: Huang Ying Date: Mon, 28 Jun 2021 19:37:09 -0700 Subject: [PATCH] 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/20210520073301.1676294-1-ying.huang@intel.com Signed-off-by: "Huang, Ying" Cc: Daniel Jordan Cc: Dan Carpenter Cc: Andrea Parri Cc: Peter Zijlstra (Intel) Cc: Andi Kleen Cc: Dave Hansen Cc: Omar Sandoval Cc: Paul McKenney Cc: Tejun Heo Cc: Will Deacon Cc: Miaohe Lin Cc: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/swapfile.c | 15 ++++++--------- 1 file changed, 6 insertions(+), 9 deletions(-) diff --git a/mm/swapfile.c b/mm/swapfile.c index 115f0b0..e898c87 100644 --- a/mm/swapfile.c +++ b/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_swap_info(void) } 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]; -- 2.7.4