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

Darrick J. Wong
2023-08-24T23:22:10Z

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);
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.
@@ -2093,47 +2100,6 @@ xfs_inodegc_queue(
}
}

-/*
- * Fold the dead CPU inodegc queue into the current CPUs queue.
- */
-void
-xfs_inodegc_cpu_dead(
- struct xfs_mount *mp,
- unsigned int dead_cpu)
-{
- struct xfs_inodegc *dead_gc, *gc;
- struct llist_node *first, *last;
- unsigned int count = 0;
-
- dead_gc = per_cpu_ptr(mp->m_inodegc, dead_cpu);
- cancel_delayed_work_sync(&dead_gc->work);
-
- if (llist_empty(&dead_gc->list))
- return;
-
- first = dead_gc->list.first;
- last = first;
- while (last->next) {
- last = last->next;
- count++;
- }
- dead_gc->list.first = NULL;
- dead_gc->items = 0;
-
- /* Add pending work to current CPU */
- gc = get_cpu_ptr(mp->m_inodegc);
- llist_add_batch(first, last, &gc->list);
- count += READ_ONCE(gc->items);
- WRITE_ONCE(gc->items, count);
-
- if (xfs_is_inodegc_enabled(mp)) {
- trace_xfs_inodegc_queue(mp, __return_address);
- mod_delayed_work_on(current_cpu(), mp->m_inodegc_wq, &gc->work,
- 0);
- }
- put_cpu_ptr(gc);
-}
-
/*
* We set the inode flag atomically with the radix tree tag. Once we get tag
* lookups on the radix tree, this inode flag can go away.
@@ -2195,7 +2161,7 @@ xfs_inodegc_shrinker_count(
if (!xfs_is_inodegc_enabled(mp))
return 0;

- 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))
return XFS_INODEGC_SHRINKER_COUNT;
@@ -2220,7 +2186,7 @@ xfs_inodegc_shrinker_scan(

trace_xfs_inodegc_shrinker_scan(mp, sc, __return_address);

- 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)) {
unsigned int h = READ_ONCE(gc->shrinker_hits);
diff --git a/fs/xfs/xfs_icache.h b/fs/xfs/xfs_icache.h
index 2fa6f2e09d07..905944dafbe5 100644
--- a/fs/xfs/xfs_icache.h
+++ b/fs/xfs/xfs_icache.h
@@ -79,7 +79,6 @@ void xfs_inodegc_push(struct xfs_mount *mp);
int xfs_inodegc_flush(struct xfs_mount *mp);
void xfs_inodegc_stop(struct xfs_mount *mp);
void xfs_inodegc_start(struct xfs_mount *mp);
-void xfs_inodegc_cpu_dead(struct xfs_mount *mp, unsigned int cpu);
int xfs_inodegc_register_shrinker(struct xfs_mount *mp);

#endif
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index a25eece3be2b..f4a8879ba0e9 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -60,6 +60,7 @@ struct xfs_error_cfg {
* Per-cpu deferred inode inactivation GC lists.
*/
struct xfs_inodegc {
+ struct xfs_mount *mp;
struct llist_head list;
struct delayed_work work;
int error;
@@ -67,9 +68,7 @@ struct xfs_inodegc {
/* approximate count of inodes in the list */
unsigned int items;
unsigned int shrinker_hits;
-#if defined(DEBUG) || defined(XFS_WARN)
unsigned int cpu;
-#endif
};

/*
@@ -249,6 +248,9 @@ typedef struct xfs_mount {
unsigned int *m_errortag;
struct xfs_kobj m_errortag_kobj;
#endif
+
+ /* cpus that have inodes queued for inactivation */
+ struct cpumask m_inodegc_cpumask;
} xfs_mount_t;

#define M_IGEO(mp) (&(mp)->m_ino_geo)
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index ef7775657ce3..6a4f8b2f6159 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1110,9 +1110,8 @@ xfs_inodegc_init_percpu(

for_each_possible_cpu(cpu) {
gc = per_cpu_ptr(mp->m_inodegc, cpu);
-#if defined(DEBUG) || defined(XFS_WARN)
gc->cpu = cpu;
-#endif
+ gc->mp = mp;
init_llist_head(&gc->list);
gc->items = 0;
gc->error = 0;
@@ -2312,7 +2311,6 @@ xfs_cpu_dead(
spin_lock(&xfs_mount_list_lock);
list_for_each_entry_safe(mp, n, &xfs_mount_list, m_mount_list) {
spin_unlock(&xfs_mount_list_lock);
- xfs_inodegc_cpu_dead(mp, cpu);
spin_lock(&xfs_mount_list_lock);
}
spin_unlock(&xfs_mount_list_lock);