Re: [PATCH 2/3] xfs: use per-mount cpumask to track nonempty percpu inodegc lists

Dave Chinner
2023-08-25T00:16:00Z

On Thu, Aug 24, 2023 at 04:21:25PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> Directly track which CPUs have contributed to the inodegc percpu lists
> instead of trusting the cpu online mask. This eliminates a theoretical
> problem where the inodegc flush functions might fail to flush a CPU's
> inodes if that CPU happened to be dying at exactly the same time. Most
> likely nobody's noticed this because the CPU dead hook moves the percpu
> inodegc list to another CPU and schedules that worker immediately. But
> it's quite possible that this is a subtle race leading to UAF if the
> inodegc flush were part of an unmount.
>
> Further benefits: This reduces the overhead of the inodegc flush code
> slightly by allowing us to ignore CPUs that have empty lists. Better
> yet, it reduces our dependence on the cpu online masks, which have been
> the cause of confusion and drama lately.
>
> Fixes: ab23a7768739 ("xfs: per-cpu deferred inode inactivation queues")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
> fs/xfs/xfs_icache.c | 60 +++++++++++----------------------------------------
> fs/xfs/xfs_icache.h | 1 -
> fs/xfs/xfs_mount.h | 6 +++--
> fs/xfs/xfs_super.c | 4 +--
> 4 files changed, 18 insertions(+), 53 deletions(-)
>
>
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index e541f5c0bc25..7fd876e94ecb 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -443,7 +443,7 @@ xfs_inodegc_queue_all(
> int cpu;
> bool ret = false;
>
> - for_each_online_cpu(cpu) {
> + for_each_cpu(cpu, &mp->m_inodegc_cpumask) {
> gc = per_cpu_ptr(mp->m_inodegc, cpu);
> if (!llist_empty(&gc->list)) {
> mod_delayed_work_on(cpu, mp->m_inodegc_wq, &gc->work, 0);
> @@ -463,7 +463,7 @@ xfs_inodegc_wait_all(
> int error = 0;
>
> flush_workqueue(mp->m_inodegc_wq);
> - for_each_online_cpu(cpu) {
> + for_each_cpu(cpu, &mp->m_inodegc_cpumask) {
> struct xfs_inodegc *gc;
>
> gc = per_cpu_ptr(mp->m_inodegc, cpu);
> @@ -1845,10 +1845,12 @@ xfs_inodegc_worker(
> struct xfs_inodegc, work);
> struct llist_node *node = llist_del_all(&gc->list);
> struct xfs_inode *ip, *n;
> + struct xfs_mount *mp = gc->mp;
> unsigned int nofs_flag;
>
> ASSERT(gc->cpu == smp_processor_id());
>
> + cpumask_test_and_clear_cpu(gc->cpu, &mp->m_inodegc_cpumask);

Why does this need to be a test-and-clear operation? If it is set,
we clear it. If it is not set, clearing it is a no-op. Hence we
don't need to test whether the bit is set first. Also,
cpumask_clear_cpu() uses clear_bit(), which is an atomic operation,
so clearing the bit isn't going to race with any other updates.

As it is, we probably want acquire semantics for the gc structure
here (see below), so I think this likely should be:

/*
* Clear the cpu mask bit and ensure that we have seen the
* latest update of the gc structure associated with this
* CPU. This matches with the release semantics used when
* setting the cpumask bit in xfs_inodegc_queue.
*/
cpumask_clear_cpu(gc->cpu, &mp->m_inodegc_cpumask);
smp_mb__after_atomic();

> WRITE_ONCE(gc->items, 0);
>
> if (!node)
> @@ -1862,7 +1864,7 @@ xfs_inodegc_worker(
> nofs_flag = memalloc_nofs_save();
>
> ip = llist_entry(node, struct xfs_inode, i_gclist);
> - trace_xfs_inodegc_worker(ip->i_mount, READ_ONCE(gc->shrinker_hits));
> + trace_xfs_inodegc_worker(mp, READ_ONCE(gc->shrinker_hits));
>
> WRITE_ONCE(gc->shrinker_hits, 0);
> llist_for_each_entry_safe(ip, n, node, i_gclist) {
> @@ -2057,6 +2059,7 @@ xfs_inodegc_queue(
> struct xfs_inodegc *gc;
> int items;
> unsigned int shrinker_hits;
> + unsigned int cpu_nr;
> unsigned long queue_delay = 1;
>
> trace_xfs_inode_set_need_inactive(ip);
> @@ -2064,12 +2067,16 @@ xfs_inodegc_queue(
> ip->i_flags |= XFS_NEED_INACTIVE;
> spin_unlock(&ip->i_flags_lock);
>
> - gc = get_cpu_ptr(mp->m_inodegc);
> + cpu_nr = get_cpu();
> + gc = this_cpu_ptr(mp->m_inodegc);
> llist_add(&ip->i_gclist, &gc->list);
> items = READ_ONCE(gc->items);
> WRITE_ONCE(gc->items, items + 1);
> shrinker_hits = READ_ONCE(gc->shrinker_hits);
>
> + if (!cpumask_test_cpu(cpu_nr, &mp->m_inodegc_cpumask))
> + cpumask_test_and_set_cpu(cpu_nr, &mp->m_inodegc_cpumask);
> +
> /*
> * We queue the work while holding the current CPU so that the work
> * is scheduled to run on this CPU.

I think we need release/acquire memory ordering on this atomic bit
set now. i.e. to guarantee that if the worker sees the cpumask bit
set (with acquire semantics), it will always see the latest item
added to the list. i.e.

/*
* Ensure the list add is always seen by anyone that
* find the cpumask bit set. This effectively gives
* the cpumask bit set operation release ordering semantics.
*/
smp_mb__before_atomic();
if (!cpumask_test_cpu(cpu_nr, &mp->m_inodegc_cpumask))
cpumask_test_and_set_cpu(cpu_nr, &mp->m_inodegc_cpumask);

Also, same comment about put_cpu() vs put_cpu_var() as the last patch.

Otherwise this seems sane.

-Dave.

--
Dave Chinner
david@fromorbit.com