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Date: Mon, 7 Jun 2021 11:38:51 +0100
From: Will Deacon <will@...nel.org>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Daniel Jordan <daniel.m.jordan@...cle.com>,
Dan Carpenter <dan.carpenter@...cle.com>,
Andrea Parri <andrea.parri@...rulasolutions.com>,
Peter Zijlstra <peterz@...radead.org>,
Andi Kleen <ak@...ux.intel.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Omar Sandoval <osandov@...com>,
Paul McKenney <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>,
Will Deacon <will.deacon@....com>,
Miaohe Lin <linmiaohe@...wei.com>
Subject: Re: [PATCH -V2] mm, swap: Remove unnecessary smp_rmb() in
swap_type_to_swap_info()
On Mon, Jun 07, 2021 at 09:49:35AM +0800, Huang, Ying wrote:
> Hi, Will,
>
> Will Deacon <will@...nel.org> writes:
>
> > On Thu, May 20, 2021 at 03:33:01PM +0800, Huang Ying wrote:
> >> 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().
> >>
> >> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> >> Cc: Daniel Jordan <daniel.m.jordan@...cle.com>
> >> Cc: Dan Carpenter <dan.carpenter@...cle.com>
> >> Cc: Andrea Parri <andrea.parri@...rulasolutions.com>
> >> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> >> Cc: Andi Kleen <ak@...ux.intel.com>
> >> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> >> Cc: Omar Sandoval <osandov@...com>
> >> Cc: Paul McKenney <paulmck@...nel.org>
> >> Cc: Tejun Heo <tj@...nel.org>
> >> Cc: Will Deacon <will.deacon@....com>
> >> Cc: Miaohe Lin <linmiaohe@...wei.com>
> >>
> >> v2:
> >>
> >> - Revise the patch description and comments per Peter's comments.
> >>
> >> ---
> >> mm/swapfile.c | 15 ++++++---------
> >> 1 file changed, 6 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >> index 2aad85751991..65dd979a0f94 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)
> >> @@ -2884,14 +2883,12 @@ 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++;
> >
> > Although I like this change, I comment you are removing refers to some
> > dodgy-looking code. For example, swap_start() has this loop:
> >
> > for (type = 0; (si = swap_type_to_swap_info(type)); type++) {
> > if (!(si->flags & SWP_USED) || !si->swap_map)
> > continue;
> >
> > so won't this just end up dereferencing NULL if nr_swapfiles < MAX_SWAPFILES?
>
> for (type = 0; (si = swap_type_to_swap_info(type)); type++) {
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Because this is the second sub-statement inside "for ()", I think that "si"
> will be checked to be non-NULL before executing the statements inside
> "{}" follows "for ()"?
Sorry, yes, you're right. I misread the loop condition.
Will
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