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Date:   Wed, 2 Dec 2020 23:08:26 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Mike Galbraith <efault@gmx.de>,
        Vitaly Wool <vitaly.wool@konsulko.com>
Cc:     Oleksandr Natalenko <oleksandr@natalenko.name>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-rt-users@vger.kernel.org
Subject: Re: scheduling while atomic in z3fold
Message-ID: <20201202220826.5chy56mbgvrwmg3d@linutronix.de>
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On 2020-12-02 03:30:27 [+0100], Mike Galbraith wrote:
> > > In an LTP install, ./runltp -f mm.  Shortly after box starts swapping
> > > insanely, it explodes quite reliably here with either z3fold or
> > > zsmalloc.. but not with zbud.
>=20
> What I'm seeing is the below.  rt_mutex_has_waiters() says yup we have
> a waiter, rt_mutex_top_waiter() emits the missing cached leftmost, and
> rt_mutex_dequeue_pi() chokes on it.  Lock is buggered.

correct. So this:

diff --git a/mm/z3fold.c b/mm/z3fold.c
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -342,7 +342,7 @@ static inline void free_handle(unsigned long handle)
=20
 	if (is_free) {
 		struct z3fold_pool *pool =3D slots_to_pool(slots);
-
+		memset(slots, 0xee, sizeof(struct z3fold_buddy_slots));
 		kmem_cache_free(pool->c_handle, slots);
 	}
 }
@@ -548,8 +549,10 @@ static void __release_z3fold_page(struct z3fold_header=
 *zhdr, bool locked)
 		set_bit(HANDLES_ORPHANED, &zhdr->slots->pool);
 	read_unlock(&zhdr->slots->lock);
=20
-	if (is_free)
+	if (is_free) {
+		memset(zhdr->slots, 0xdd, sizeof(struct z3fold_buddy_slots));
 		kmem_cache_free(pool->c_handle, zhdr->slots);
+	}
=20
 	if (locked)
 		z3fold_page_unlock(zhdr);

resulted in:

|[  377.200696] Out of memory: Killed process 284358 (oom01) total-vm:15780=
488kB, anon-rss:150624kB, file-rss:0kB, shmem-rss:0kB, UID:0 pgtables:16760=
kB oom_score_adj:0
|[  377.205438] ------------[ cut here ]------------
|[  377.205441] pvqspinlock: lock 0xffff8880105c6828 has corrupted value 0x=
dddddddd!
|[  377.205448] WARNING: CPU: 6 PID: 72 at kernel/locking/qspinlock_paravir=
t.h:498 __pv_queued_spin_unlock_slowpath+0xb3/0xc0
|[  377.205455] Modules linked in:
|[  377.205456] CPU: 6 PID: 72 Comm: oom_reaper Not tainted 5.10.0-rc6-rt13=
-rt+ #103
|[  377.205458] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.=
14.0-1 04/01/2014
|[  377.205460] RIP: 0010:__pv_queued_spin_unlock_slowpath+0xb3/0xc0
=E2=80=A6
|[  377.205475] Call Trace:
|[  377.205477]  __raw_callee_save___pv_queued_spin_unlock_slowpath+0x11/0x=
20
|[  377.205481]  .slowpath+0x9/0xe
|[  377.205483]  _raw_spin_unlock_irqrestore+0x5/0x50
|[  377.205486]  rt_mutex_futex_unlock+0x9e/0xb0
|[  377.205488]  z3fold_free+0x2b0/0x470
|[  377.205491]  zswap_free_entry+0x7d/0xc0
|[  377.205493]  zswap_frontswap_invalidate_page+0x87/0x90
|[  377.205495]  __frontswap_invalidate_page+0x58/0x90
|[  377.205496]  swap_range_free.constprop.0+0x99/0xb0
|[  377.205499]  swapcache_free_entries+0x131/0x390
|[  377.205501]  free_swap_slot+0x99/0xc0
|[  377.205502]  __swap_entry_free+0x8a/0xa0
|[  377.205504]  free_swap_and_cache+0x36/0xd0
|[  377.205506]  zap_pte_range+0x16a/0x940
|[  377.205509]  unmap_page_range+0x1d8/0x310
|[  377.205514]  __oom_reap_task_mm+0xe7/0x190
|[  377.205520]  oom_reap_task_mm+0x5a/0x280
|[  377.205521]  oom_reaper+0x98/0x1c0
|[  377.205525]  kthread+0x18c/0x1b0
|[  377.205528]  ret_from_fork+0x22/0x30
|[  377.205531] ---[ end trace 0000000000000002 ]---

Then I reverted commit
   4a3ac9311dac3 ("mm/z3fold.c: add inter-page compaction")

and it seems to work now. Any suggestions? It looks like use-after-free.

Sebastian