From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com [115.124.30.130]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8A05F2904 for ; Tue, 5 Aug 2025 07:53:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.130 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754380443; cv=none; b=aslwGh69fQ02LQ7Q9QC0PUNt8rXxdOI8MqHOnfGaqGfOVy2+qUwBz0dd/knI1OzJJv9pQr6COI/7dNRJH/AzS+1mKDRY+/370xSS9GI6paDoOQH0soxBy/JyvOioK6OaXSc3ztEqm/0e3OS628KCN4RdIMAbI7qcJjlrp4IVABQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754380443; c=relaxed/simple; bh=tWGYh/6V8yFHh7sK2cuMQidlURY8Xa7NmtuDc043muo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iAYzryUFZrWgwG4AylI/LAR5Df/S41v4NUU9kyjD5pURnNArXwYNQeLjGgqbINB2vf27XmzeRzOKpaGzOp4RrxrGkr1l1xt5zEz8N6bPqxVcuWVSSrkhML4wOj0Uxefo4NHGuOqrGX4LsefsHhmWiunILtRC0EhASvpZl1Te51U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=sQ8ZZx7V; arc=none smtp.client-ip=115.124.30.130 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="sQ8ZZx7V" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1754380431; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=/nQryawfyDuIJRKhkainkKo5d5VP9AzO/w40vwouXNI=; b=sQ8ZZx7VrjPHL48uTaJKhE93A2TxIblg/SigI96q2OeXmOLTTH/ukX/F82LeUEcZTXVG7AwRVME2BWzpir3CCHR5w4IKEuIGjO3n4l9ir0ipaKrUJyepCdd1oHcoe3kYXxcpnzsT6wFOXirzpmFINiYaFSJ6ysvN8wVNguwmWBs= Received: from 30.74.144.114(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0Wl4MJeu_1754380429 cluster:ay36) by smtp.aliyun-inc.com; Tue, 05 Aug 2025 15:53:49 +0800 Message-ID: Date: Tue, 5 Aug 2025 15:53:48 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: Fix the race between collapse and PT_RECLAIM under per-vma lock To: Qi Zheng , Barry Song <21cnbao@gmail.com>, akpm@linux-foundation.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Barry Song , "Lai, Yi" , David Hildenbrand , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Suren Baghdasaryan , Lokesh Gidra , Tangquan Zheng , Lance Yang , Zi Yan , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain References: <20250805035447.7958-1-21cnbao@gmail.com> <35417160-86bf-4580-8ae9-5cadd4f6401d@bytedance.com> From: Baolin Wang In-Reply-To: <35417160-86bf-4580-8ae9-5cadd4f6401d@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2025/8/5 14:42, Qi Zheng wrote: > Hi Barry, > > On 8/5/25 11:54 AM, Barry Song wrote: >> From: Barry Song >> >> 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. > > There is indeed a race condition here. And after applying this patch, I > can no longer reproduce the problem locally (I was able to reproduce it > stably locally last night). > > But I still can't figure out how this race condtion causes the > following panic: > > exit_mmap > --> mmap_read_lock() >     unmap_vmas() >     --> pte_offset_map_lock >         --> rcu_read_lock() >             check if the pmd entry is a PTE page >             ptl = pte_lockptr(mm, &pmdval)  <-- ptl is NULL >             spin_lock(ptl)                  <-- PANIC!! > > If this PTE page is freed by pt_reclaim (via RCU), then the ptl can not > be NULL. > > The collapse holds mmap write lock, so it is impossible to be concurrent > with exit_mmap(). > > Confusing. :( IIUC, the issue is not caused by the concurrency between exit_mmap and collapse, but rather by the concurrency between pt_reclaim and collapse. Before this patch, khugepaged might incorrectly restore a PTE pagetable that had already been freed. pt_reclaim has cleared the pmd entry and freed the PTE page table. However, due to the race condition, check_pmd_still_valid() still passes and continues to attempt the collapse: _pmd = pmdp_collapse_flush(vma, address, pmd); ---> returns a none pmd entry (the original pmd entry has been cleared) pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); ---> returns pte == NULL Then khugepaged will restore the old PTE pagetable with an invalid pmd entry: pmd_populate(mm, pmd, pmd_pgtable(_pmd)); So when the process exits and trys to free the mapping of the process, traversing the invalid pmd table will lead to a crash. Barry, please correct me if I have misunderstood something. >>   [   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]  >>   [   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" >> Reported-by: "Lai, Yi" >> Closes: https://lore.kernel.org/all/aJAFrYfyzGpbm+0m@ly-workstation/ >> Cc: David Hildenbrand >> Cc: Lorenzo Stoakes >> Cc: Qi Zheng >> Cc: Vlastimil Babka >> Cc: Jann Horn >> Cc: Suren Baghdasaryan >> Cc: Lokesh Gidra >> Cc: Tangquan Zheng >> Cc: Lance Yang >> Cc: Zi Yan >> Cc: Baolin Wang >> Cc: Liam R. Howlett >> Cc: Nico Pache >> Cc: Ryan Roberts >> Cc: Dev Jain >> Signed-off-by: Barry Song >> --- >>   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,