Re: [PATCH v2] mm/hugetlb: fix possible recursive locking detected warning

[Muchun Song]

> On Jul 12, 2024, at 11:13, Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
> 
> When tries to demote 1G hugetlb folios, a lockdep warning is observed:
> 
> ============================================
> WARNING: possible recursive locking detected
> 6.10.0-rc6-00452-ga4d0275fa660-dirty #79 Not tainted
> --------------------------------------------
> bash/710 is trying to acquire lock:
> ffffffff8f0a7850 (&h->resize_lock){+.+.}-{3:3}, at: demote_store+0x244/0x460
> 
> but task is already holding lock:
> ffffffff8f0a6f48 (&h->resize_lock){+.+.}-{3:3}, at: demote_store+0xae/0x460
> 
> other info that might help us debug this:
> Possible unsafe locking scenario:
> 
>       CPU0
>       ----
>  lock(&h->resize_lock);
>  lock(&h->resize_lock);
> 
> *** DEADLOCK ***
> 
> May be due to missing lock nesting notation
> 
> 4 locks held by bash/710:
> #0: ffff8f118439c3f0 (sb_writers#5){.+.+}-{0:0}, at: ksys_write+0x64/0xe0
> #1: ffff8f11893b9e88 (&of->mutex#2){+.+.}-{3:3}, at: kernfs_fop_write_iter+0xf8/0x1d0
> #2: ffff8f1183dc4428 (kn->active#98){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x100/0x1d0
> #3: ffffffff8f0a6f48 (&h->resize_lock){+.+.}-{3:3}, at: demote_store+0xae/0x460
> 
> stack backtrace:
> CPU: 3 PID: 710 Comm: bash Not tainted 6.10.0-rc6-00452-ga4d0275fa660-dirty #79
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl+0x68/0xa0
> __lock_acquire+0x10f2/0x1ca0
> lock_acquire+0xbe/0x2d0
> __mutex_lock+0x6d/0x400
> demote_store+0x244/0x460
> kernfs_fop_write_iter+0x12c/0x1d0
> vfs_write+0x380/0x540
> ksys_write+0x64/0xe0
> do_syscall_64+0xb9/0x1d0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fa61db14887
> RSP: 002b:00007ffc56c48358 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007fa61db14887
> RDX: 0000000000000002 RSI: 000055a030050220 RDI: 0000000000000001
> RBP: 000055a030050220 R08: 00007fa61dbd1460 R09: 000000007fffffff
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000002
> R13: 00007fa61dc1b780 R14: 00007fa61dc17600 R15: 00007fa61dc16a00
> </TASK>
> 
> Lockdep considers this an AA deadlock because the different resize_lock
> mutexes reside in the same lockdep class, but this is a false positive.
> Place them in distinct classes to avoid these warnings.
> 
> Fixes: 8531fc6f52f5 ("hugetlb: add hugetlb demote page support")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

Thanks.