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Message-ID: <20250513093448.592150-1-gavinguo@igalia.com>
Date: Tue, 13 May 2025 17:34:48 +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] 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 solution here is to unlock the pagecache_folio and provide the
pagecache_folio_unlocked variable to the caller to have the visibility
over the pagecache_folio status for subsequent handling.
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>
Signed-off-by: Gavin Guo <gavinguo@...lia.com>
---
mm/hugetlb.c | 33 ++++++++++++++++++++++++++++-----
1 file changed, 28 insertions(+), 5 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e3e6ac991b9c..ad54a74aa563 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6115,7 +6115,8 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
* Keep the pte_same checks anyway to make transition from the mutex easier.
*/
static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
- struct vm_fault *vmf)
+ struct vm_fault *vmf,
+ bool *pagecache_folio_unlocked)
{
struct vm_area_struct *vma = vmf->vma;
struct mm_struct *mm = vma->vm_mm;
@@ -6212,6 +6213,22 @@ 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 and we won't re-lock it in hugetlb_wp(). The
+ * pagecache_folio could be truncated after being
+ * unlocked. So its state should not be relied
+ * subsequently.
+ *
+ * Setting *pagecache_folio_unlocked to true allows the
+ * caller to handle any necessary logic related to the
+ * folio's unlocked state.
+ */
+ if (pagecache_folio) {
+ folio_unlock(pagecache_folio);
+ if (pagecache_folio_unlocked)
+ *pagecache_folio_unlocked = true;
+ }
/*
* Drop hugetlb_fault_mutex and vma_lock before
* unmapping. unmapping needs to hold vma_lock
@@ -6566,7 +6583,7 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping,
hugetlb_count_add(pages_per_huge_page(h), mm);
if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
/* Optimization, do the COW without a second fault */
- ret = hugetlb_wp(folio, vmf);
+ ret = hugetlb_wp(folio, vmf, NULL);
}
spin_unlock(vmf->ptl);
@@ -6638,6 +6655,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
struct hstate *h = hstate_vma(vma);
struct address_space *mapping;
int need_wait_lock = 0;
+ bool pagecache_folio_unlocked = false;
struct vm_fault vmf = {
.vma = vma,
.address = address & huge_page_mask(h),
@@ -6792,7 +6810,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
if (flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)) {
if (!huge_pte_write(vmf.orig_pte)) {
- ret = hugetlb_wp(pagecache_folio, &vmf);
+ ret = hugetlb_wp(pagecache_folio, &vmf,
+ &pagecache_folio_unlocked);
goto out_put_page;
} else if (likely(flags & FAULT_FLAG_WRITE)) {
vmf.orig_pte = huge_pte_mkdirty(vmf.orig_pte);
@@ -6809,10 +6828,14 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
out_ptl:
spin_unlock(vmf.ptl);
- if (pagecache_folio) {
+ /*
+ * If the pagecache_folio is unlocked in hugetlb_wp(), we skip
+ * folio_unlock() here.
+ */
+ if (pagecache_folio && !pagecache_folio_unlocked)
folio_unlock(pagecache_folio);
+ if (pagecache_folio)
folio_put(pagecache_folio);
- }
out_mutex:
hugetlb_vma_unlock_read(vma);
base-commit: d76bb1ebb5587f66b0f8b8099bfbb44722bc08b3
--
2.43.0
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