From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 D375A2661AC for ; Mon, 24 Feb 2025 18:04:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740420248; cv=none; b=st891Z8tsBxoEnjX+mDYY+RyFDw1cGrY+rc2/ZSnJ66m1htpptuoqeWPYPk3Cyupyd2hmkxyj12F7uK0beZebLIqLtELgVV79NnfH0JPgXp8jA3QDiKWULmDq6fD82iBsIvVxVTk3fujVOSPuhuJP6NUZna5WmUfLESFIkpq/6Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740420248; c=relaxed/simple; bh=C0oe1Ya2EZLRxlAQ0L8g/GFjD5Xv4b7G7R1GBfyoduM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=l/rKW2Y1f0poaXtPOsOKJr6kpqqAiUqU7Y7kgnPuKSF0Ee6KI3ghWhCNBqSZns/fqe+me6ReU1HUjHy4lgFYY02i4aYpet0cRSkrJlnpep+IRTtDcRuAsvCTnO7zSe7OfWRFiPLuxa6WiqYtNPAF9OObPYJxwRRfHrfm9wjhLH4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=WLx3QF0g; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="WLx3QF0g" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1740420245; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=FKhJDB5I9/Akcs9qeehefKWvAKLmf9oiYnEk3pKbOsI=; b=WLx3QF0ga8wL9kBgVs+R9BRLward/UM1PpjlaUBwqUX06j1HXxNTYWTCV9pRe45jUYtt1t DSnl+hZPKU4ijBJ5+NfjW+tTcr2UbJVFl6bxb/gjKt2+Ib9uDkWZ++cHN5srbcSJjy5ssO YMM4Qntrsr4TMyBXHxKeqrraE9fvyJE= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-477-_55KaSc8N7CcUjuw0fM_mA-1; Mon, 24 Feb 2025 13:04:03 -0500 X-MC-Unique: _55KaSc8N7CcUjuw0fM_mA-1 X-Mimecast-MFC-AGG-ID: _55KaSc8N7CcUjuw0fM_mA_1740420242 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-43943bd1409so33160235e9.3 for ; Mon, 24 Feb 2025 10:04:03 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1740420242; x=1741025042; h=content-transfer-encoding:in-reply-to:organization:autocrypt :content-language:from:references:cc:to:subject:user-agent :mime-version:date:message-id:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=FKhJDB5I9/Akcs9qeehefKWvAKLmf9oiYnEk3pKbOsI=; b=wwqj5ymVQPQB3qGeihs2E8pU+5vaafM2VPb1INbeMyK379ARfEHTJHRTBQqij6w4ny o7qHLQaDaYGGSiIMesWR1uaPJUdUBrO6K8eb4yavzzNpE39agVyL6lgeHA89h2eqdSCd 3gBNsmoOmUf2Y0MeHM7C7D0MaI5m/MlWU0JSosP0ELXB+mzeQy/hyXgB5TDvrHkaMNbt kEhjQqkgUoP4NvWTbBQyQOzd8b4f39p6msVEgDVLdo3M4qdx2M/SSYnlW6Gqv2AMqnFN FrJMi+secsp1vkGFlOs20Fm7iE5RCKZJRZWkepwWmkM0576uyKVkwTphn+3Uyuast4OK sYcA== X-Forwarded-Encrypted: i=1; AJvYcCUCuIHq5a/fuwE30cHFOCVUY6lJrkncmJGbAya81MN/tVWHFWLAtbSCrcfeSYZ/STPD0PTvQeY=@vger.kernel.org X-Gm-Message-State: AOJu0YwH+zYSmy7olZrIcehZy4KTZA4M7Jk8WG6/fUdINwlbHDPQx1C4 JdMVdl6nvD6aDbMNql6D6vbjO70HYet3MPQr/f7YO6+zJhnQKTBgGg9K0Yy+nvr/gfPqEFau22L 1XRKTtP8rQIwRk+JGD1/oIQFvShZK4fU8n2ksptq/J2PaCzDiPYmy5w== X-Gm-Gg: ASbGncuaukzrcyEQUrqg3kA/eqHaPgue/kXXiPgyPMUdykyKzzdOe4qt6L08WtPWKM+ 9CZnfU5bPfo6UfobmcOLeuz57TDi/KvCl6y15DeP1fjNSwQGxgLWuLOEUVaHxmOqQ2b8MTCsfdu 9+74PvbiBvl9MZPBN/A7lRol4HcyZZzVeSlkKzeOInPSBTCKB9Rquk/aG+JCw1hjj9ZDJ6W0WvP bRU1oNAWY7XKlFDwc7Vrf+TM2Qvj2Nj/uFrp45iWA1Ttt5kcknGW+rpMVSJV0hODaFF7px/yzk5 mGtng5hbsAy0HZPibtssGk3FefngmomDsY88IeK/90Fi5Q== X-Received: by 2002:a05:600c:3c9f:b0:439:34dd:c3cc with SMTP id 5b1f17b1804b1-439ae2197d1mr99189705e9.22.1740420241922; Mon, 24 Feb 2025 10:04:01 -0800 (PST) X-Google-Smtp-Source: AGHT+IH0qisyK2TmyiUHxqGDOo+VFtcO9MSPSIVtTFeVJRq9q6wtNqxhOQqdWsjAGVkrBH4KkE69kA== X-Received: by 2002:a05:600c:3c9f:b0:439:34dd:c3cc with SMTP id 5b1f17b1804b1-439ae2197d1mr99188955e9.22.1740420241360; Mon, 24 Feb 2025 10:04:01 -0800 (PST) Received: from [192.168.3.141] (p4ff234b6.dip0.t-ipconnect.de. [79.242.52.182]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-439b02f252dsm111389435e9.22.2025.02.24.10.03.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2025 10:04:00 -0800 (PST) Message-ID: Date: Mon, 24 Feb 2025 19:03:58 +0100 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache To: Peter Xu , Barry Song <21cnbao@gmail.com> Cc: Liam.Howlett@oracle.com, aarcange@redhat.com, akpm@linux-foundation.org, axelrasmussen@google.com, bgeffon@google.com, brauner@kernel.org, hughd@google.com, jannh@google.com, kaleshsingh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lokeshgidra@google.com, mhocko@suse.com, ngeoffray@google.com, rppt@kernel.org, ryan.roberts@arm.com, shuah@kernel.org, surenb@google.com, v-songbaohua@oppo.com, viro@zeniv.linux.org.uk, willy@infradead.org, zhangpeng362@huawei.com, zhengtangquan@oppo.com, yuzhao@google.com, stable@vger.kernel.org References: <69dbca2b-cf67-4fd8-ba22-7e6211b3e7c4@redhat.com> <20250220092101.71966-1-21cnbao@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 24.02.25 18:50, Peter Xu wrote: > On Sun, Feb 23, 2025 at 10:31:37AM +1300, Barry Song wrote: >> On Fri, Feb 21, 2025 at 2:49 PM Peter Xu wrote: >>> >>> On Fri, Feb 21, 2025 at 01:07:24PM +1300, Barry Song wrote: >>>> On Fri, Feb 21, 2025 at 12:32 PM Peter Xu wrote: >>>>> >>>>> On Thu, Feb 20, 2025 at 10:21:01PM +1300, Barry Song wrote: >>>>>> 2. src_anon_vma and its lock – swapcache doesn’t require it（folio is not mapped） >>>>> >>>>> Could you help explain what guarantees the rmap walk not happen on a >>>>> swapcache page? >>>>> >>>>> I'm not familiar with this path, though at least I see damon can start a >>>>> rmap walk on PageAnon almost with no locking.. some explanations would be >>>>> appreciated. >>>> >>>> I am observing the following in folio_referenced(), which the anon_vma lock >>>> was originally intended to protect. >>>> >>>> if (!pra.mapcount) >>>> return 0; >>>> >>>> I assume all other rmap walks should do the same? >>> >>> Yes normally there'll be a folio_mapcount() check, however.. >>> >>>> >>>> int folio_referenced(struct folio *folio, int is_locked, >>>> struct mem_cgroup *memcg, unsigned long *vm_flags) >>>> { >>>> >>>> bool we_locked = false; >>>> struct folio_referenced_arg pra = { >>>> .mapcount = folio_mapcount(folio), >>>> .memcg = memcg, >>>> }; >>>> >>>> struct rmap_walk_control rwc = { >>>> .rmap_one = folio_referenced_one, >>>> .arg = (void *)&pra, >>>> .anon_lock = folio_lock_anon_vma_read, >>>> .try_lock = true, >>>> .invalid_vma = invalid_folio_referenced_vma, >>>> }; >>>> >>>> *vm_flags = 0; >>>> if (!pra.mapcount) >>>> return 0; >>>> ... >>>> } >>>> >>>> By the way, since the folio has been under reclamation in this case and >>>> isn't in the lru, this should also prevent the rmap walk, right? >>> >>> .. I'm not sure whether it's always working. >>> >>> The thing is anon doesn't even require folio lock held during (1) checking >>> mapcount and (2) doing the rmap walk, in all similar cases as above. I see >>> nothing blocks it from a concurrent thread zapping that last mapcount: >>> >>> thread 1 thread 2 >>> -------- -------- >>> [whatever scanner] >>> check folio_mapcount(), non-zero >>> zap the last map.. then mapcount==0 >>> rmap_walk() >>> >>> Not sure if I missed something. >>> >>> The other thing is IIUC swapcache page can also have chance to be faulted >>> in but only if a read not write. I actually had a feeling that your >>> reproducer triggered that exact path, causing a read swap in, reusing the >>> swapcache page, and hit the sanity check there somehow (even as mentioned >>> in the other reply, I don't yet know why the 1st check didn't seem to >>> work.. as we do check folio->index twice..). >>> >>> Said that, I'm not sure if above concern will happen in this specific case, >>> as UIFFDIO_MOVE is pretty special, that we check exclusive bit first in swp >>> entry so we know it's definitely not mapped elsewhere, meanwhile if we hold >>> pgtable lock so maybe it can't get mapped back.. it is just still tricky, >>> at least we do some dances all over releasing and retaking locks. >>> >>> We could either justify that's safe, or maybe still ok and simpler if we >>> could take anon_vma write lock, making sure nobody will be able to read the >>> folio->index when it's prone to an update. >> >> What prompted me to do the former is that folio_get_anon_vma() returns >> NULL for an unmapped folio. As for the latter, we need to carefully evaluate >> whether the change below is safe. >> >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -505,7 +505,7 @@ struct anon_vma *folio_get_anon_vma(const struct >> folio *folio) >> anon_mapping = (unsigned long)READ_ONCE(folio->mapping); >> if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) >> goto out; >> >> - if (!folio_mapped(folio)) >> + if (!folio_mapped(folio) && !folio_test_swapcache(folio)) >> goto out; >> >> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); >> @@ -521,7 +521,7 @@ struct anon_vma *folio_get_anon_vma(const struct >> folio *folio) >> * SLAB_TYPESAFE_BY_RCU guarantees that - so the atomic_inc_not_zero() >> * above cannot corrupt). >> */ > > [1] > >> >> - if (!folio_mapped(folio)) { >> + if (!folio_mapped(folio) && !folio_test_swapcache(folio)) { >> rcu_read_unlock(); >> put_anon_vma(anon_vma); >> return NULL; > > Hmm, this let me go back read again on how we manage anon_vma lifespan, > then I just noticed this may not work. > > See the comment right above [1], here's a full version: > > /* > * If this folio is still mapped, then its anon_vma cannot have been > * freed. But if it has been unmapped, we have no security against the > * anon_vma structure being freed and reused (for another anon_vma: > * SLAB_TYPESAFE_BY_RCU guarantees that - so the atomic_inc_not_zero() > * above cannot corrupt). > */ > > So afaiu that means we pretty much very rely upon folio_mapped() check to > make sure anon_vma being valid at all that we fetched from folio->mapping, > not to mention the rmap walk later afterwards. > > Then above diff in folio_get_anon_vma() should be problematic, as when > "folio_mapped()==false && folio_test_swapcache()==true", above change will > start to return anon_vma pointer even if the anon_vma could have been freed > and reused by other VMAs. When splitting a folio, we use folio_get_anon_vma(). That seems to work as long as we have the folio locked. -- Cheers, David / dhildenb