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Message-ID: <20250514064729.GA17622@system.software.com>
Date: Wed, 14 May 2025 15:47:29 +0900
From: Byungchul Park <byungchul@...com>
To: Gavin Guo <gavinguo@...lia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, muchun.song@...ux.dev,
osalvador@...e.de, akpm@...ux-foundation.org,
mike.kravetz@...cle.com, kernel-dev@...lia.com,
stable@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
Florent Revest <revest@...gle.com>, Gavin Shan <gshan@...hat.com>,
kernel_team@...ynix.com
Subject: Re: [PATCH] mm/hugetlb: fix a deadlock with pagecache_folio and
hugetlb_fault_mutex_table
On Tue, May 13, 2025 at 05:34:48PM +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
Hi,
I'm trying to reproduce using the test program [1]. But not yet
produced. I see a lot of segfaults while running [1]. I guess
something goes wrong. Is there any prerequisite condition to reproduce
it? Lemme know if any. Or can you try DEPT15 with your config and
environment by the following steps:
1. Apply the patchset on v6.15-rc6.
https://lkml.kernel.org/r/20250513100730.12664-1-byungchul@sk.com
2. Turn on CONFIG_DEPT.
3. Run test program reproducing the deadlock.
4. Check dmesg to see if dept reported the dependency.
Byungchul
> [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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