Re: [PATCH] bcachefs: Use alloc_percpu_gfp to avoid deadlock

[Dennis Zhou <dennis@kernel.org>]

Hello,

On Thu, Feb 20, 2025 at 03:37:26PM -0500, Kent Overstreet wrote:
> On Thu, Feb 20, 2025 at 06:16:43PM +0100, Vlastimil Babka wrote:
> > On 2/20/25 11:57, Alan Huang wrote:
> > > Ping
> > > 
> > >> On Feb 12, 2025, at 22:27, Kent Overstreet <kent.overstreet@linux.dev> wrote:
> > >> 
> > >> Adding pcpu people to the CC
> > >> 
> > >> On Wed, Feb 12, 2025 at 06:06:25PM +0800, Alan Huang wrote:
> > >>> The cycle:
> > >>> 
> > >>> CPU0: CPU1:
> > >>> bc->lock pcpu_alloc_mutex
> > >>> pcpu_alloc_mutex bc->lock
> > >>> 
> > >>> Reported-by: syzbot+fe63f377148a6371a9db@syzkaller.appspotmail.com
> > >>> Tested-by: syzbot+fe63f377148a6371a9db@syzkaller.appspotmail.com
> > >>> Signed-off-by: Alan Huang <mmpgouride@gmail.com>
> > >> 
> > >> So pcpu_alloc_mutex -> fs_reclaim?
> > >> 
> > >> That's really awkward; seems like something that might invite more
> > >> issues. We can apply your fix if we need to, but I want to hear with the
> > >> percpu people have to say first.
> > >> 
> > >> ======================================================
> > >> WARNING: possible circular locking dependency detected
> > >> 6.14.0-rc2-syzkaller-00039-g09fbf3d50205 #0 Not tainted
> > >> ------------------------------------------------------
> > >> syz.0.21/5625 is trying to acquire lock:
> > >> ffffffff8ea19608 (pcpu_alloc_mutex){+.+.}-{4:4}, at: pcpu_alloc_noprof+0x293/0x1760 mm/percpu.c:1782
> > >> 
> > >> but task is already holding lock:
> > >> ffff888051401c68 (&bc->lock){+.+.}-{4:4}, at: bch2_btree_node_mem_alloc+0x559/0x16f0 fs/bcachefs/btree_cache.c:804
> > >> 
> > >> which lock already depends on the new lock.
> > >> 
> > >> 
> > >> the existing dependency chain (in reverse order) is:
> > >> 
> > >> -> #2 (&bc->lock){+.+.}-{4:4}:
> > >>       lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5851
> > >>       __mutex_lock_common kernel/locking/mutex.c:585 [inline]
> > >>       __mutex_lock+0x19c/0x1010 kernel/locking/mutex.c:730
> > >>       bch2_btree_cache_scan+0x184/0xec0 fs/bcachefs/btree_cache.c:482
> > >>       do_shrink_slab+0x72d/0x1160 mm/shrinker.c:437
> > >>       shrink_slab+0x1093/0x14d0 mm/shrinker.c:664
> > >>       shrink_one+0x43b/0x850 mm/vmscan.c:4868
> > >>       shrink_many mm/vmscan.c:4929 [inline]
> > >>       lru_gen_shrink_node mm/vmscan.c:5007 [inline]
> > >>       shrink_node+0x37c5/0x3e50 mm/vmscan.c:5978
> > >>       kswapd_shrink_node mm/vmscan.c:6807 [inline]
> > >>       balance_pgdat mm/vmscan.c:6999 [inline]
> > >>       kswapd+0x20f3/0x3b10 mm/vmscan.c:7264
> > >>       kthread+0x7a9/0x920 kernel/kthread.c:464
> > >>       ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:148
> > >>       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> > >> 
> > >> -> #1 (fs_reclaim){+.+.}-{0:0}:
> > >>       lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5851
> > >>       __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:4066 [inline]
> > >>       slab_alloc_node mm/slub.c:4144 [inline]
> > >>       __do_kmalloc_node mm/slub.c:4293 [inline]
> > >>       __kmalloc_noprof+0xae/0x4c0 mm/slub.c:4306
> > >>       kmalloc_noprof include/linux/slab.h:905 [inline]
> > >>       kzalloc_noprof include/linux/slab.h:1037 [inline]
> > >>       pcpu_mem_zalloc mm/percpu.c:510 [inline]
> > >>       pcpu_alloc_chunk mm/percpu.c:1430 [inline]
> > >>       pcpu_create_chunk+0x57/0xbc0 mm/percpu-vm.c:338
> > >>       pcpu_balance_populated mm/percpu.c:2063 [inline]
> > >>       pcpu_balance_workfn+0xc4d/0xd40 mm/percpu.c:2200
> > >>       process_one_work kernel/workqueue.c:3236 [inline]
> > >>       process_scheduled_works+0xa66/0x1840 kernel/workqueue.c:3317
> > >>       worker_thread+0x870/0xd30 kernel/workqueue.c:3398
> > >>       kthread+0x7a9/0x920 kernel/kthread.c:464
> > >>       ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:148
> > >>       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> > 
> > Seeing this as part of the chain (fs reclaim from a worker doing
> > pcpu_balance_workfn) makes me think Michal's patch could be a fix to this:
> > 
> > https://lore.kernel.org/all/20250206122633.167896-1-mhocko@kernel.org/
> 
> Thanks for the link - that does look like just the thing.

Sorry I missed the first email asking to weigh in.

Michal's problem is a little bit different than what's happening here.
He's having an issue where a alloc_percpu_gfp(NOFS/NOIO) is considered
atomic and failing during probing. This is because we don't have enough
percpu memory backed to fulfill the "atomic" requests.

Historically we've considered any allocation that's not GFP_KERNEL to be
atomic. Here it seems like the alloc_percpu() behind the bc->lock()
should have been an "atomic" allocation to prevent the lock cycle?

Thanks,
Dennis