@@ -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 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
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();
}
/*********************************
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. Reported-by: syzbot <syzkaller@googlegroups.com> Signed-off-by: Yosry Ahmed <yosryahmed@google.com> --- Andrew, I am not sure what's the best way to handle this. This fix is already merged into Linus's tree and had CC:stable, so I thought it's best to revert it and replace it with a separate fix, especially that functionally the new fix is completely different anyway. Does the revert automatically signal the stable maintainers to drop it? Do we need to add CC:stable to this revert as well? --- mm/zswap.c | 19 +++---------------- 1 file changed, 3 insertions(+), 16 deletions(-)