Message-ID: <20250521115727.2202284-1-gavinguo@igalia.com>
Date: Wed, 21 May 2025 19:57:27 +0800
From: Gavin Guo <gavinguo@...lia.com>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	muchun.song@...ux.dev,
	osalvador@...e.de,
	akpm@...ux-foundation.org,
	mike.kravetz@...cle.com,
	kernel-dev@...lia.com,
	Gavin Guo <gavinguo@...lia.com>,
	stable@...r.kernel.org,
	Hugh Dickins <hughd@...gle.com>,
	Florent Revest <revest@...gle.com>,
	Gavin Shan <gshan@...hat.com>
Subject: [PATCH v2] mm/hugetlb: fix a deadlock with pagecache_folio and hugetlb_fault_mutex_table

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@...r.kernel.org
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Florent Revest <revest@...gle.com>
Cc: Gavin Shan <gshan@...hat.com>
Suggested-by: Oscar Salvador <osalvador@...e.de>
Signed-off-by: Gavin Guo <gavinguo@...lia.com>
---
V1 -> V2
Suggested-by Oscar Salvador:
  - Use folio_test_locked to replace the unnecessary parameter passing.

 mm/hugetlb.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7ae38bfb9096..ed501f134eff 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6226,6 +6226,12 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
 			u32 hash;
 
 			folio_put(old_folio);
+			/*
+			 * The pagecache_folio needs to be unlocked to avoid
+			 * deadlock when the child unmaps the folio.
+			 */
+			if (pagecache_folio)
+				folio_unlock(pagecache_folio);
 			/*
 			 * Drop hugetlb_fault_mutex and vma_lock before
 			 * unmapping.  unmapping needs to hold vma_lock
@@ -6823,8 +6829,13 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 out_ptl:
 	spin_unlock(vmf.ptl);
 
+	/*
+	 * hugetlb_wp() might have already unlocked pagecache_folio, so
+	 * skip it if that is the case.
+	 */
 	if (pagecache_folio) {
-		folio_unlock(pagecache_folio);
+		if (folio_test_locked(pagecache_folio))
+			folio_unlock(pagecache_folio);
 		folio_put(pagecache_folio);
 	}
 out_mutex:

base-commit: 4a95bc121ccdaee04c4d72f84dbfa6b880a514b6
-- 
2.43.0

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