Re: [PATCH] mm: Fix the race between collapse and PT_RECLAIM under per-vma lock

[David Hildenbrand <david@redhat.com>]

On 05.08.25 05:54, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> The check_pmd_still_valid() call during collapse is currently only
> protected by the mmap_lock in write mode, which was sufficient when
> pt_reclaim always ran under mmap_lock in read mode. However, since
> madvise_dontneed can now execute under a per-VMA lock, this assumption
> is no longer valid. As a result, a race condition can occur between
> collapse and PT_RECLAIM, potentially leading to a kernel panic.
> 
>   [   38.151897] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] SMP KASI
>   [   38.153519] KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
>   [   38.154605] CPU: 0 UID: 0 PID: 721 Comm: repro Not tainted 6.16.0-next-20250801-next-2025080 #1 PREEMPT(voluntary)
>   [   38.155929] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org4
>   [   38.157418] RIP: 0010:kasan_byte_accessible+0x15/0x30
>   [   38.158125] Code: 03 0f 1f 40 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 0f 1f 00 48 b8 00 00 00 00 00 fc0
>   [   38.160461] RSP: 0018:ffff88800feef678 EFLAGS: 00010286
>   [   38.161220] RAX: dffffc0000000000 RBX: 0000000000000001 RCX: 1ffffffff0dde60c
>   [   38.162232] RDX: 0000000000000000 RSI: ffffffff85da1e18 RDI: dffffc0000000003
>   [   38.163176] RBP: ffff88800feef698 R08: 0000000000000001 R09: 0000000000000000
>   [   38.164195] R10: 0000000000000000 R11: ffff888016a8ba58 R12: 0000000000000018
>   [   38.165189] R13: 0000000000000018 R14: ffffffff85da1e18 R15: 0000000000000000
>   [   38.166100] FS:  0000000000000000(0000) GS:ffff8880e3b40000(0000) knlGS:0000000000000000
>   [   38.167137] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   [   38.167891] CR2: 00007f97fadfe504 CR3: 0000000007088005 CR4: 0000000000770ef0
>   [   38.168812] PKRU: 55555554
>   [   38.169275] Call Trace:
>   [   38.169647]  <TASK>
>   [   38.169975]  ? __kasan_check_byte+0x19/0x50
>   [   38.170581]  lock_acquire+0xea/0x310
>   [   38.171083]  ? rcu_is_watching+0x19/0xc0
>   [   38.171615]  ? __sanitizer_cov_trace_const_cmp4+0x1a/0x20
>   [   38.172343]  ? __sanitizer_cov_trace_const_cmp8+0x1c/0x30
>   [   38.173130]  _raw_spin_lock+0x38/0x50
>   [   38.173707]  ? __pte_offset_map_lock+0x1a2/0x3c0
>   [   38.174390]  __pte_offset_map_lock+0x1a2/0x3c0
>   [   38.174987]  ? __pfx___pte_offset_map_lock+0x10/0x10
>   [   38.175724]  ? __pfx_pud_val+0x10/0x10
>   [   38.176308]  ? __sanitizer_cov_trace_const_cmp1+0x1e/0x30
>   [   38.177183]  unmap_page_range+0xb60/0x43e0
>   [   38.177824]  ? __pfx_unmap_page_range+0x10/0x10
>   [   38.178485]  ? mas_next_slot+0x133a/0x1a50
>   [   38.179079]  unmap_single_vma.constprop.0+0x15b/0x250
>   [   38.179830]  unmap_vmas+0x1fa/0x460
>   [   38.180373]  ? __pfx_unmap_vmas+0x10/0x10
>   [   38.180994]  ? __sanitizer_cov_trace_const_cmp4+0x1a/0x20
>   [   38.181877]  exit_mmap+0x1a2/0xb40
>   [   38.182396]  ? lock_release+0x14f/0x2c0
>   [   38.182929]  ? __pfx_exit_mmap+0x10/0x10
>   [   38.183474]  ? __pfx___mutex_unlock_slowpath+0x10/0x10
>   [   38.184188]  ? mutex_unlock+0x16/0x20
>   [   38.184704]  mmput+0x132/0x370
>   [   38.185208]  do_exit+0x7e7/0x28c0
>   [   38.185682]  ? __this_cpu_preempt_check+0x21/0x30
>   [   38.186328]  ? do_group_exit+0x1d8/0x2c0
>   [   38.186873]  ? __pfx_do_exit+0x10/0x10
>   [   38.187401]  ? __this_cpu_preempt_check+0x21/0x30
>   [   38.188036]  ? _raw_spin_unlock_irq+0x2c/0x60
>   [   38.188634]  ? lockdep_hardirqs_on+0x89/0x110
>   [   38.189313]  do_group_exit+0xe4/0x2c0
>   [   38.189831]  __x64_sys_exit_group+0x4d/0x60
>   [   38.190413]  x64_sys_call+0x2174/0x2180
>   [   38.190935]  do_syscall_64+0x6d/0x2e0
>   [   38.191449]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> This patch moves the vma_start_write() call to precede
> check_pmd_still_valid(), ensuring that the check is also properly
> protected by the per-VMA lock.
> 
> Fixes: a6fde7add78d ("mm: use per_vma lock for MADV_DONTNEED")
> Tested-by: "Lai, Yi" <yi1.lai@linux.intel.com>
> Reported-by: "Lai, Yi" <yi1.lai@linux.intel.com>
> Closes: https://lore.kernel.org/all/aJAFrYfyzGpbm+0m@ly-workstation/
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Jann Horn <jannh@google.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Lokesh Gidra <lokeshgidra@google.com>
> Cc: Tangquan Zheng <zhengtangquan@oppo.com>
> Cc: Lance Yang <ioworker0@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>   mm/khugepaged.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 374a6a5193a7..6b40bdfd224c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1172,11 +1172,11 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>   	if (result != SCAN_SUCCEED)
>   		goto out_up_write;
>   	/* check if the pmd is still valid */
> +	vma_start_write(vma);
>   	result = check_pmd_still_valid(mm, address, pmd);
>   	if (result != SCAN_SUCCEED)
>   		goto out_up_write;
>   
> -	vma_start_write(vma);
>   	anon_vma_lock_write(vma->anon_vma);
>   
>   	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,

LGTM, I was wondering whether we should just place it next to the 
mmap_write_lock() with the assumption that hugepage_vma_revalidate() 
will commonly not fail.

So personally, I would move it further up.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb