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Date:   Thu, 3 Dec 2020 14:39:34 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Mike Galbraith <efault@gmx.de>
Cc:     Vitaly Wool <vitaly.wool@konsulko.com>,
        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: <20201203133934.37aotbdjnd36lrxv@linutronix.de>
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On 2020-12-03 09:18:21 [+0100], Mike Galbraith wrote:
> On Thu, 2020-12-03 at 03:16 +0100, Mike Galbraith wrote:
> > On Wed, 2020-12-02 at 23:08 +0100, Sebastian Andrzej Siewior wrote:
> > Looks like...
> >
> > d8f117abb380 z3fold: fix use-after-free when freeing handles
> >
> > ...wasn't completely effective...
> 
> The top two hunks seem to have rendered the thing RT tolerant.

Yes, it appears to. I have no idea if this is a proper fix or not.
Without your write lock, after a few attempts, KASAN says:

| BUG: KASAN: use-after-free in __pv_queued_spin_lock_slowpath+0x293/0x770
| Write of size 2 at addr ffff88800e0e10aa by task kworker/u16:3/237
| 
| CPU: 5 PID: 237 Comm: kworker/u16:3 Tainted: G        W         5.10.0-rc6-rt13-rt+
| Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-1 04/01/2014
| Workqueue: zswap1 compact_page_work
| Call Trace:
|  dump_stack+0x7d/0xa3
|  print_address_description.constprop.0+0x19/0x120
|  __kasan_report.cold+0x1d/0x35
|  kasan_report+0x3a/0x50
|  check_memory_region+0x145/0x1a0
|  __pv_queued_spin_lock_slowpath+0x293/0x770
|  _raw_spin_lock_irq+0xca/0xe0
|  rt_read_unlock+0x2c/0x70
|  __release_z3fold_page.constprop.0+0x45e/0x620
|  do_compact_page+0x674/0xa50
|  process_one_work+0x63a/0x1130
|  worker_thread+0xd3/0xc80
|  kthread+0x401/0x4e0
|  ret_from_fork+0x22/0x30
| 
| Allocated by task 225 (systemd-journal):
|  kasan_save_stack+0x1b/0x40
|  __kasan_kmalloc.constprop.0+0xc2/0xd0
|  kmem_cache_alloc+0x103/0x2b0
|  z3fold_alloc+0x597/0x1970
|  zswap_frontswap_store+0x928/0x1bf0
|  __frontswap_store+0x117/0x320
|  swap_writepage+0x34/0x70
|  pageout+0x268/0x7c0
|  shrink_page_list+0x13e1/0x1e80
|  shrink_inactive_list+0x303/0xde0
|  shrink_lruvec+0x3dd/0x660
|  shrink_node_memcgs+0x3a1/0x600
|  shrink_node+0x3a7/0x1350
|  shrink_zones+0x1f1/0x7f0
|  do_try_to_free_pages+0x219/0xcc0
|  try_to_free_pages+0x1c5/0x4b0
|  __perform_reclaim+0x18f/0x2c0
|  __alloc_pages_slowpath.constprop.0+0x7ea/0x1790
|  __alloc_pages_nodemask+0x5f5/0x700
|  page_cache_ra_unbounded+0x30f/0x690
|  do_sync_mmap_readahead+0x3e3/0x640
|  filemap_fault+0x981/0x1110
|  __xfs_filemap_fault+0x12d/0x840
|  __do_fault+0xf3/0x4b0
|  do_fault+0x202/0x8c0
|  __handle_mm_fault+0x338/0x500
|  handle_mm_fault+0x1a8/0x670
|  do_user_addr_fault+0x409/0x8b0
|  exc_page_fault+0x60/0xc0
|  asm_exc_page_fault+0x1e/0x30
| 
| Freed by task 71 (oom_reaper):
|  kasan_save_stack+0x1b/0x40
|  kasan_set_track+0x1c/0x30
|  kasan_set_free_info+0x1b/0x30
|  __kasan_slab_free+0x110/0x150
|  kmem_cache_free+0x7f/0x450
|  z3fold_free+0x1f8/0xc90
|  zswap_free_entry+0x168/0x230
|  zswap_frontswap_invalidate_page+0x145/0x190
|  __frontswap_invalidate_page+0xe8/0x1a0
|  swap_range_free.constprop.0+0x266/0x300
|  swapcache_free_entries+0x1dc/0x970
|  free_swap_slot+0x19c/0x290
|  __swap_entry_free+0x139/0x160
|  free_swap_and_cache+0xda/0x230
|  zap_pte_range+0x275/0x1590
|  unmap_page_range+0x320/0x690
|  __oom_reap_task_mm+0x207/0x330
|  oom_reap_task_mm+0x78/0x7e0
|  oom_reap_task+0x6d/0x1a0
|  oom_reaper+0x103/0x290
|  kthread+0x401/0x4e0
|  ret_from_fork+0x22/0x30
| 
| The buggy address belongs to the object at ffff88800e0e1080
|  which belongs to the cache z3fold_handle of size 88
| The buggy address is located 42 bytes inside of
|  88-byte region [ffff88800e0e1080, ffff88800e0e10d8)
| The buggy address belongs to the page:
| page:000000002ba661bc refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0xe0e1
| flags: 0xffff800000200(slab)
| raw: 000ffff800000200 dead000000000100 dead000000000122 ffff88800aa4eb40
| raw: 0000000000000000 0000000000200020 00000001ffffffff 0000000000000000
| page dumped because: kasan: bad access detected
| 
| Memory state around the buggy address:
|  ffff88800e0e0f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
|  ffff88800e0e1000: 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc
| >ffff88800e0e1080: fa fb fb fb fb fb fb fb fb fb fb fc fc fc fc fc
|                                   ^
|  ffff88800e0e1100: 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc
|  ffff88800e0e1180: 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc
| ==================================================================
| Disabling lock debugging due to kernel taint

with the lock I haven't seen anything.

Sebastian