Re: [PATCH] Grab mm lock before grabbing pt lock

[Juergen Gross <jgross@suse.com>]

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On 04.12.24 11:35, Maksym Planeta wrote:
> Function xen_pin_page calls xen_pte_lock, which in turn grab page
> table lock (ptlock). When locking, xen_pte_lock expect mm->page_table_lock
> to be held before grabbing ptlock, but this does not happen when pinning
> is caused by xen_mm_pin_all.
> 
> This commit addresses lockdep warning below, which shows up when
> suspending a Xen VM.
> 
> [ 3680.658422] Freezing user space processes
> [ 3680.660156] Freezing user space processes completed (elapsed 0.001 seconds)
> [ 3680.660182] OOM killer disabled.
> [ 3680.660192] Freezing remaining freezable tasks
> [ 3680.661485] Freezing remaining freezable tasks completed (elapsed 0.001 seconds)
> [ 3680.685254]
> [ 3680.685265] ==================================
> [ 3680.685269] WARNING: Nested lock was not taken
> [ 3680.685274] 6.12.0+ #16 Tainted: G        W
> [ 3680.685279] ----------------------------------
> [ 3680.685283] migration/0/19 is trying to lock:
> [ 3680.685288] ffff88800bac33c0 (ptlock_ptr(ptdesc)#2){+.+.}-{3:3}, at: xen_pin_page+0x175/0x1d0
> [ 3680.685303]
> [ 3680.685303] but this task is not holding:
> [ 3680.685308] init_mm.page_table_lock
> [ 3680.685311]
> [ 3680.685311] stack backtrace:
> [ 3680.685316] CPU: 0 UID: 0 PID: 19 Comm: migration/0 Tainted: G        W          6.12.0+ #16
> [ 3680.685324] Tainted: [W]=WARN
> [ 3680.685328] Stopper: multi_cpu_stop+0x0/0x120 <- __stop_cpus.constprop.0+0x8c/0xd0
> [ 3680.685339] Call Trace:
> [ 3680.685344]  <TASK>
> [ 3680.685347]  dump_stack_lvl+0x77/0xb0
> [ 3680.685356]  __lock_acquire+0x917/0x2310
> [ 3680.685364]  lock_acquire+0xce/0x2c0
> [ 3680.685369]  ? xen_pin_page+0x175/0x1d0
> [ 3680.685373]  _raw_spin_lock_nest_lock+0x2f/0x70
> [ 3680.685381]  ? xen_pin_page+0x175/0x1d0
> [ 3680.685386]  xen_pin_page+0x175/0x1d0
> [ 3680.685390]  ? __pfx_xen_pin_page+0x10/0x10
> [ 3680.685394]  __xen_pgd_walk+0x233/0x2c0
> [ 3680.685401]  ? stop_one_cpu+0x91/0x100
> [ 3680.685405]  __xen_pgd_pin+0x5d/0x250
> [ 3680.685410]  xen_mm_pin_all+0x70/0xa0
> [ 3680.685415]  xen_pv_pre_suspend+0xf/0x280
> [ 3680.685420]  xen_suspend+0x57/0x1a0
> [ 3680.685428]  multi_cpu_stop+0x6b/0x120
> [ 3680.685432]  ? update_cpumasks_hier+0x7c/0xa60
> [ 3680.685439]  ? __pfx_multi_cpu_stop+0x10/0x10
> [ 3680.685443]  cpu_stopper_thread+0x8c/0x140
> [ 3680.685448]  ? smpboot_thread_fn+0x20/0x1f0
> [ 3680.685454]  ? __pfx_smpboot_thread_fn+0x10/0x10
> [ 3680.685458]  smpboot_thread_fn+0xed/0x1f0
> [ 3680.685462]  kthread+0xde/0x110
> [ 3680.685467]  ? __pfx_kthread+0x10/0x10
> [ 3680.685471]  ret_from_fork+0x2f/0x50
> [ 3680.685478]  ? __pfx_kthread+0x10/0x10
> [ 3680.685482]  ret_from_fork_asm+0x1a/0x30
> [ 3680.685489]  </TASK>
> [ 3680.685491]
> [ 3680.685491] other info that might help us debug this:
> [ 3680.685497] 1 lock held by migration/0/19:
> [ 3680.685500]  #0: ffffffff8284df38 (pgd_lock){+.+.}-{3:3}, at: xen_mm_pin_all+0x14/0xa0
> [ 3680.685512]
> [ 3680.685512] stack backtrace:
> [ 3680.685518] CPU: 0 UID: 0 PID: 19 Comm: migration/0 Tainted: G        W          6.12.0+ #16
> [ 3680.685528] Tainted: [W]=WARN
> [ 3680.685531] Stopper: multi_cpu_stop+0x0/0x120 <- __stop_cpus.constprop.0+0x8c/0xd0
> [ 3680.685538] Call Trace:
> [ 3680.685541]  <TASK>
> [ 3680.685544]  dump_stack_lvl+0x77/0xb0
> [ 3680.685549]  __lock_acquire+0x93c/0x2310
> [ 3680.685554]  lock_acquire+0xce/0x2c0
> [ 3680.685558]  ? xen_pin_page+0x175/0x1d0
> [ 3680.685562]  _raw_spin_lock_nest_lock+0x2f/0x70
> [ 3680.685568]  ? xen_pin_page+0x175/0x1d0
> [ 3680.685572]  xen_pin_page+0x175/0x1d0
> [ 3680.685578]  ? __pfx_xen_pin_page+0x10/0x10
> [ 3680.685582]  __xen_pgd_walk+0x233/0x2c0
> [ 3680.685588]  ? stop_one_cpu+0x91/0x100
> [ 3680.685592]  __xen_pgd_pin+0x5d/0x250
> [ 3680.685596]  xen_mm_pin_all+0x70/0xa0
> [ 3680.685600]  xen_pv_pre_suspend+0xf/0x280
> [ 3680.685607]  xen_suspend+0x57/0x1a0
> [ 3680.685611]  multi_cpu_stop+0x6b/0x120
> [ 3680.685615]  ? update_cpumasks_hier+0x7c/0xa60
> [ 3680.685620]  ? __pfx_multi_cpu_stop+0x10/0x10
> [ 3680.685625]  cpu_stopper_thread+0x8c/0x140
> [ 3680.685629]  ? smpboot_thread_fn+0x20/0x1f0
> [ 3680.685634]  ? __pfx_smpboot_thread_fn+0x10/0x10
> [ 3680.685638]  smpboot_thread_fn+0xed/0x1f0
> [ 3680.685642]  kthread+0xde/0x110
> [ 3680.685645]  ? __pfx_kthread+0x10/0x10
> [ 3680.685649]  ret_from_fork+0x2f/0x50
> [ 3680.685654]  ? __pfx_kthread+0x10/0x10
> [ 3680.685657]  ret_from_fork_asm+0x1a/0x30
> [ 3680.685662]  </TASK>
> [ 3680.685267] xen:grant_table: Grant tables using version 1 layout
> [ 3680.685921] OOM killer enabled.
> [ 3680.685934] Restarting tasks ... done.
> 
> Signed-off-by: Maksym Planeta <maksym@exostellar.io>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

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