Re: [PATCH v2] mm/hugetlb: fix a deadlock with pagecache_folio and hugetlb_fault_mutex_table

[Oscar Salvador <osalvador@suse.de>]

On Wed, May 21, 2025 at 07:57:27PM +0800, Gavin Guo wrote:
> The patch fixes a deadlock which can be triggered by an internal
> syzkaller [1] reproducer and captured by bpftrace script [2] and its log
> [3] in this scenario:
> 
> Process 1                              Process 2
> ---				       ---
> hugetlb_fault
>   mutex_lock(B) // take B
>   filemap_lock_hugetlb_folio
>     filemap_lock_folio
>       __filemap_get_folio
>         folio_lock(A) // take A
>   hugetlb_wp
>     mutex_unlock(B) // release B
>     ...                                hugetlb_fault
>     ...                                  mutex_lock(B) // take B
>                                          filemap_lock_hugetlb_folio
>                                            filemap_lock_folio
>                                              __filemap_get_folio
>                                                folio_lock(A) // blocked
>     unmap_ref_private
>     ...
>     mutex_lock(B) // retake and blocked
> 
> This is a ABBA deadlock involving two locks:
> - Lock A: pagecache_folio lock
> - Lock B: hugetlb_fault_mutex_table lock
> 
> The deadlock occurs between two processes as follows:
> 1. The first process (let’s call it Process 1) is handling a
> copy-on-write (COW) operation on a hugepage via hugetlb_wp. Due to
> insufficient reserved hugetlb pages, Process 1, owner of the reserved
> hugetlb page, attempts to unmap a hugepage owned by another process
> (non-owner) to satisfy the reservation. Before unmapping, Process 1
> acquires lock B (hugetlb_fault_mutex_table lock) and then lock A
> (pagecache_folio lock). To proceed with the unmap, it releases Lock B
> but retains Lock A. After the unmap, Process 1 tries to reacquire Lock
> B. However, at this point, Lock B has already been acquired by another
> process.
> 
> 2. The second process (Process 2) enters the hugetlb_fault handler
> during the unmap operation. It successfully acquires Lock B
> (hugetlb_fault_mutex_table lock) that was just released by Process 1,
> but then attempts to acquire Lock A (pagecache_folio lock), which is
> still held by Process 1.
> 
> As a result, Process 1 (holding Lock A) is blocked waiting for Lock B
> (held by Process 2), while Process 2 (holding Lock B) is blocked waiting
> for Lock A (held by Process 1), constructing a ABBA deadlock scenario.
> 
> The error message:
> INFO: task repro_20250402_:13229 blocked for more than 64 seconds.
>       Not tainted 6.15.0-rc3+ #24
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:repro_20250402_ state:D stack:25856 pid:13229 tgid:13228 ppid:3513   task_flags:0x400040 flags:0x00004006
> Call Trace:
>  <TASK>
>  __schedule+0x1755/0x4f50
>  schedule+0x158/0x330
>  schedule_preempt_disabled+0x15/0x30
>  __mutex_lock+0x75f/0xeb0
>  hugetlb_wp+0xf88/0x3440
>  hugetlb_fault+0x14c8/0x2c30
>  trace_clock_x86_tsc+0x20/0x20
>  do_user_addr_fault+0x61d/0x1490
>  exc_page_fault+0x64/0x100
>  asm_exc_page_fault+0x26/0x30
> RIP: 0010:__put_user_4+0xd/0x20
>  copy_process+0x1f4a/0x3d60
>  kernel_clone+0x210/0x8f0
>  __x64_sys_clone+0x18d/0x1f0
>  do_syscall_64+0x6a/0x120
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> RIP: 0033:0x41b26d
>  </TASK>
> INFO: task repro_20250402_:13229 is blocked on a mutex likely owned by task repro_20250402_:13250.
> task:repro_20250402_ state:D stack:28288 pid:13250 tgid:13228 ppid:3513   task_flags:0x400040 flags:0x00000006
> Call Trace:
>  <TASK>
>  __schedule+0x1755/0x4f50
>  schedule+0x158/0x330
>  io_schedule+0x92/0x110
>  folio_wait_bit_common+0x69a/0xba0
>  __filemap_get_folio+0x154/0xb70
>  hugetlb_fault+0xa50/0x2c30
>  trace_clock_x86_tsc+0x20/0x20
>  do_user_addr_fault+0xace/0x1490
>  exc_page_fault+0x64/0x100
>  asm_exc_page_fault+0x26/0x30
> RIP: 0033:0x402619
>  </TASK>
> INFO: task repro_20250402_:13250 blocked for more than 65 seconds.
>       Not tainted 6.15.0-rc3+ #24
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:repro_20250402_ state:D stack:28288 pid:13250 tgid:13228 ppid:3513   task_flags:0x400040 flags:0x00000006
> Call Trace:
>  <TASK>
>  __schedule+0x1755/0x4f50
>  schedule+0x158/0x330
>  io_schedule+0x92/0x110
>  folio_wait_bit_common+0x69a/0xba0
>  __filemap_get_folio+0x154/0xb70
>  hugetlb_fault+0xa50/0x2c30
>  trace_clock_x86_tsc+0x20/0x20
>  do_user_addr_fault+0xace/0x1490
>  exc_page_fault+0x64/0x100
>  asm_exc_page_fault+0x26/0x30
> RIP: 0033:0x402619
>  </TASK>
> 
> Showing all locks held in the system:
> 1 lock held by khungtaskd/35:
>  #0: ffffffff879a7440 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x30/0x180
> 2 locks held by repro_20250402_/13229:
>  #0: ffff888017d801e0 (&mm->mmap_lock){++++}-{4:4}, at: lock_mm_and_find_vma+0x37/0x300
>  #1: ffff888000fec848 (&hugetlb_fault_mutex_table[i]){+.+.}-{4:4}, at: hugetlb_wp+0xf88/0x3440
> 3 locks held by repro_20250402_/13250:
>  #0: ffff8880177f3d08 (vm_lock){++++}-{0:0}, at: do_user_addr_fault+0x41b/0x1490
>  #1: ffff888000fec848 (&hugetlb_fault_mutex_table[i]){+.+.}-{4:4}, at: hugetlb_fault+0x3b8/0x2c30
>  #2: ffff8880129500e8 (&resv_map->rw_sema){++++}-{4:4}, at: hugetlb_fault+0x494/0x2c30
> 
> Link: https://drive.google.com/file/d/1DVRnIW-vSayU5J1re9Ct_br3jJQU6Vpb/view?usp=drive_link [1]
> Link: https://github.com/bboymimi/bpftracer/blob/master/scripts/hugetlb_lock_debug.bt [2]
> Link: https://drive.google.com/file/d/1bWq2-8o-BJAuhoHWX7zAhI6ggfhVzQUI/view?usp=sharing [3]
> Fixes: 40549ba8f8e0 ("hugetlb: use new vma_lock for pmd sharing synchronization")
> Cc: stable@vger.kernel.org
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Florent Revest <revest@google.com>
> Cc: Gavin Shan <gshan@redhat.com>
> Suggested-by: Oscar Salvador <osalvador@suse.de>
> Signed-off-by: Gavin Guo <gavinguo@igalia.com>

Acked-by: Oscar Salvador <osalvador@suse.de>



-- 
Oscar Salvador
SUSE Labs