The patch titled Subject: mm/hugetlb: fix a deadlock with pagecache_folio and hugetlb_fault_mutex_table has been added to the -mm mm-new branch. Its filename is mm-hugetlb-fix-a-deadlock-with-pagecache_folio-and-hugetlb_fault_mutex_table.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress patches, and acceptance into mm-new is a notification for others take notice and to finish up reviews. Please do not hesitate to respond to review feedback and post updated versions to replace or incrementally fixup patches in mm-new.

Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days

------------------------------------------------------ From: Gavin Guo gavinguo@igalia.com Subject: mm/hugetlb: fix a deadlock with pagecache_folio and hugetlb_fault_mutex_table Date: Tue, 13 May 2025 17:34:48 +0800

Fix 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=d... [1] Link: https://github.com/bboymimi/bpftracer/blob/master/scripts/hugetlb_lock_debug... [2] Link: https://drive.google.com/file/d/1bWq2-8o-BJAuhoHWX7zAhI6ggfhVzQUI/view?usp=s... [3] Link: https://lkml.kernel.org/r/20250513093448.592150-1-gavinguo@igalia.com Fixes: 40549ba8f8e0 ("hugetlb: use new vma_lock for pmd sharing synchronization") Signed-off-by: Gavin Guo gavinguo@igalia.com Cc: Hugh Dickins hughd@google.com Cc: Florent Revest revest@google.com Cc: Gavin Shan gshan@redhat.com Cc: Muchun Song muchun.song@linux.dev Cc: Oscar Salvador osalvador@suse.de Cc: Byungchul Park byungchul@sk.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/hugetlb.c | 33 ++++++++++++++++++++++++++++----- 1 file changed, 28 insertions(+), 5 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-fix-a-deadlock-with-pagecache_folio-and-hugetlb_fault_mutex_table +++ a/mm/hugetlb.c @@ -6131,7 +6131,8 @@ static void unmap_ref_private(struct mm_ * 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; @@ -6229,6 +6230,22 @@ retry_avoidcopy:

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 * in write mode. Dropping vma_lock in read mode @@ -6581,7 +6598,7 @@ static vm_fault_t hugetlb_no_page(struct 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); @@ -6653,6 +6670,7 @@ vm_fault_t hugetlb_fault(struct mm_struc 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), @@ -6807,7 +6825,8 @@ vm_fault_t hugetlb_fault(struct mm_struc

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); @@ -6824,10 +6843,14 @@ out_put_page: 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);

_

Patches currently in -mm which might be from gavinguo@igalia.com are

mm-hugetlb-fix-a-deadlock-with-pagecache_folio-and-hugetlb_fault_mutex_table.patch