Revert "mm: zswap: fix race between [de]compression and CPU hotunplug"

This reverts commit eaebeb93922ca6ab0dd92027b73d0112701706ef.

Commit eaebeb93922c ("mm: zswap: fix race between [de]compression and CPU
hotunplug") used the CPU hotplug lock in zswap compress/decompress
operations to protect against a race with CPU hotunplug making some
per-CPU resources go away.

However, zswap compress/decompress can be reached through reclaim while
the lock is held, resulting in a potential deadlock as reported by syzbot:
======================================================
WARNING: possible circular locking dependency detected
6.13.0-rc6-syzkaller-00006-g5428dc1906dd #0 Not tainted
------------------------------------------------------
kswapd0/89 is trying to acquire lock:
 ffffffff8e7d2ed0 (cpu_hotplug_lock){++++}-{0:0}, at: acomp_ctx_get_cpu mm/zswap.c:886 [inline]
 ffffffff8e7d2ed0 (cpu_hotplug_lock){++++}-{0:0}, at: zswap_compress mm/zswap.c:908 [inline]
 ffffffff8e7d2ed0 (cpu_hotplug_lock){++++}-{0:0}, at: zswap_store_page mm/zswap.c:1439 [inline]
 ffffffff8e7d2ed0 (cpu_hotplug_lock){++++}-{0:0}, at: zswap_store+0xa74/0x1ba0 mm/zswap.c:1546

but task is already holding lock:
 ffffffff8ea355a0 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat mm/vmscan.c:6871 [inline]
 ffffffff8ea355a0 (fs_reclaim){+.+.}-{0:0}, at: kswapd+0xb58/0x2f30 mm/vmscan.c:7253

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (fs_reclaim){+.+.}-{0:0}:
        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
        __fs_reclaim_acquire mm/page_alloc.c:3853 [inline]
        fs_reclaim_acquire+0x88/0x130 mm/page_alloc.c:3867
        might_alloc include/linux/sched/mm.h:318 [inline]
        slab_pre_alloc_hook mm/slub.c:4070 [inline]
        slab_alloc_node mm/slub.c:4148 [inline]
        __kmalloc_cache_node_noprof+0x40/0x3a0 mm/slub.c:4337
        kmalloc_node_noprof include/linux/slab.h:924 [inline]
        alloc_worker kernel/workqueue.c:2638 [inline]
        create_worker+0x11b/0x720 kernel/workqueue.c:2781
        workqueue_prepare_cpu+0xe3/0x170 kernel/workqueue.c:6628
        cpuhp_invoke_callback+0x48d/0x830 kernel/cpu.c:194
        __cpuhp_invoke_callback_range kernel/cpu.c:965 [inline]
        cpuhp_invoke_callback_range kernel/cpu.c:989 [inline]
        cpuhp_up_callbacks kernel/cpu.c:1020 [inline]
        _cpu_up+0x2b3/0x580 kernel/cpu.c:1690
        cpu_up+0x184/0x230 kernel/cpu.c:1722
        cpuhp_bringup_mask+0xdf/0x260 kernel/cpu.c:1788
        cpuhp_bringup_cpus_parallel+0xf9/0x160 kernel/cpu.c:1878
        bringup_nonboot_cpus+0x2b/0x50 kernel/cpu.c:1892
        smp_init+0x34/0x150 kernel/smp.c:1009
        kernel_init_freeable+0x417/0x5d0 init/main.c:1569
        kernel_init+0x1d/0x2b0 init/main.c:1466
        ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

-> #0 (cpu_hotplug_lock){++++}-{0:0}:
        check_prev_add kernel/locking/lockdep.c:3161 [inline]
        check_prevs_add kernel/locking/lockdep.c:3280 [inline]
        validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
        __lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
        percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
        cpus_read_lock+0x42/0x150 kernel/cpu.c:490
        acomp_ctx_get_cpu mm/zswap.c:886 [inline]
        zswap_compress mm/zswap.c:908 [inline]
        zswap_store_page mm/zswap.c:1439 [inline]
        zswap_store+0xa74/0x1ba0 mm/zswap.c:1546
        swap_writepage+0x647/0xce0 mm/page_io.c:279
        shmem_writepage+0x1248/0x1610 mm/shmem.c:1579
        pageout mm/vmscan.c:696 [inline]
        shrink_folio_list+0x35ee/0x57e0 mm/vmscan.c:1374
        shrink_inactive_list mm/vmscan.c:1967 [inline]
        shrink_list mm/vmscan.c:2205 [inline]
        shrink_lruvec+0x16db/0x2f30 mm/vmscan.c:5734
        mem_cgroup_shrink_node+0x385/0x8e0 mm/vmscan.c:6575
        mem_cgroup_soft_reclaim mm/memcontrol-v1.c:312 [inline]
        memcg1_soft_limit_reclaim+0x346/0x810 mm/memcontrol-v1.c:362
        balance_pgdat mm/vmscan.c:6975 [inline]
        kswapd+0x17b3/0x2f30 mm/vmscan.c:7253
        kthread+0x2f0/0x390 kernel/kthread.c:389
        ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(fs_reclaim);
                               lock(cpu_hotplug_lock);
                               lock(fs_reclaim);
  rlock(cpu_hotplug_lock);

 *** DEADLOCK ***

1 lock held by kswapd0/89:
  #0: ffffffff8ea355a0 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat mm/vmscan.c:6871 [inline]
  #0: ffffffff8ea355a0 (fs_reclaim){+.+.}-{0:0}, at: kswapd+0xb58/0x2f30 mm/vmscan.c:7253

stack backtrace:
CPU: 0 UID: 0 PID: 89 Comm: kswapd0 Not tainted 6.13.0-rc6-syzkaller-00006-g5428dc1906dd #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Call Trace:
 <TASK>
  __dump_stack lib/dump_stack.c:94 [inline]
  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
  print_circular_bug+0x13a/0x1b0 kernel/locking/lockdep.c:2074
  check_noncircular+0x36a/0x4a0 kernel/locking/lockdep.c:2206
  check_prev_add kernel/locking/lockdep.c:3161 [inline]
  check_prevs_add kernel/locking/lockdep.c:3280 [inline]
  validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
  __lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
  lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
  percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
  cpus_read_lock+0x42/0x150 kernel/cpu.c:490
  acomp_ctx_get_cpu mm/zswap.c:886 [inline]
  zswap_compress mm/zswap.c:908 [inline]
  zswap_store_page mm/zswap.c:1439 [inline]
  zswap_store+0xa74/0x1ba0 mm/zswap.c:1546
  swap_writepage+0x647/0xce0 mm/page_io.c:279
  shmem_writepage+0x1248/0x1610 mm/shmem.c:1579
  pageout mm/vmscan.c:696 [inline]
  shrink_folio_list+0x35ee/0x57e0 mm/vmscan.c:1374
  shrink_inactive_list mm/vmscan.c:1967 [inline]
  shrink_list mm/vmscan.c:2205 [inline]
  shrink_lruvec+0x16db/0x2f30 mm/vmscan.c:5734
  mem_cgroup_shrink_node+0x385/0x8e0 mm/vmscan.c:6575
  mem_cgroup_soft_reclaim mm/memcontrol-v1.c:312 [inline]
  memcg1_soft_limit_reclaim+0x346/0x810 mm/memcontrol-v1.c:362
  balance_pgdat mm/vmscan.c:6975 [inline]
  kswapd+0x17b3/0x2f30 mm/vmscan.c:7253
  kthread+0x2f0/0x390 kernel/kthread.c:389
  ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
 </TASK>

Revert the change. A different fix for the race with CPU hotunplug will
follow.

Link: https://lkml.kernel.org/r/20250107222236.2715883-1-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Sam Sun <samsun1006219@gmail.com>
Cc: Vitaly Wool <vitalywool@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 3 insertions, 16 deletions

diff --git a/mm/zswap.c b/mm/zswap.c
index 5a27af8d86ea..f6316b66fb23 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -880,18 +880,6 @@ static int zswap_cpu_comp_dead(unsigned int cpu, struct hlist_node *node)
 	return 0;
 }
 
-/* Prevent CPU hotplug from freeing up the per-CPU acomp_ctx resources */
-static struct crypto_acomp_ctx *acomp_ctx_get_cpu(struct crypto_acomp_ctx __percpu *acomp_ctx)
-{
-	cpus_read_lock();
-	return raw_cpu_ptr(acomp_ctx);
-}
-
-static void acomp_ctx_put_cpu(void)
-{
-	cpus_read_unlock();
-}
-
 static bool zswap_compress(struct page *page, struct zswap_entry *entry,
 			   struct zswap_pool *pool)
 {
@@ -905,7 +893,8 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
 	gfp_t gfp;
 	u8 *dst;
 
-	acomp_ctx = acomp_ctx_get_cpu(pool->acomp_ctx);
+	acomp_ctx = raw_cpu_ptr(pool->acomp_ctx);
+
 	mutex_lock(&acomp_ctx->mutex);
 
 	dst = acomp_ctx->buffer;
@@ -961,7 +950,6 @@ unlock:
 		zswap_reject_alloc_fail++;
 
 	mutex_unlock(&acomp_ctx->mutex);
-	acomp_ctx_put_cpu();
 	return comp_ret == 0 && alloc_ret == 0;
 }
 
@@ -972,7 +960,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio)
 	struct crypto_acomp_ctx *acomp_ctx;
 	u8 *src;
 
-	acomp_ctx = acomp_ctx_get_cpu(entry->pool->acomp_ctx);
+	acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
 	mutex_lock(&acomp_ctx->mutex);
 
 	src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
@@ -1002,7 +990,6 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio)
 
 	if (src != acomp_ctx->buffer)
 		zpool_unmap_handle(zpool, entry->handle);
-	acomp_ctx_put_cpu();
 }
 
 /*********************************