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[2003:cb:c738:3c00:8b01:4fd9:b833:e1e9]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39c3020d6c8sm11299424f8f.73.2025.04.07.01.46.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Apr 2025 01:46:12 -0700 (PDT) Message-ID: Date: Mon, 7 Apr 2025 10:46:10 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] x86/mm/pat: fix VM_PAT handling when fork() fails in copy_page_range() To: Fedor Pchelkin , peterx@redhat.com Cc: mawupeng , linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, xrivendell7@gmail.com, wang1315768607@163.com, fleischermarius@gmail.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, akpm@linux-foundation.org References: From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: NHYOTdxgcsi60CBnb_PxN1DxcAkClGQ_VjnrqWSevQ0_1744015574 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: B5C11C0003 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: h684jwp59rh34hiwe7ud6cwxjipwxqtn X-HE-Tag: 1744015576-186121 X-HE-Meta: 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 Ijkx38rw lBCytbOup43FFEEfLMSIq/3IynCoHXwgwjODtdK8NcIYHXVB9FLrcDnCDCm9e6JWn4uDUDERiTmGSdvUeHn487ZHEOtaaxhBFbVhC9qsmpqB/eFyGES4kM+bu46AVJssnHVVQ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 07.04.25 10:43, Fedor Pchelkin wrote: > Hi, David, Peter > > Sorry for reviving an old thread. I've tried to keep the context as-is. > Here is an original link in the archives: > https://lore.kernel.org/lkml/20241029210331.1339581-1-david@redhat.com/T/#u > > Please see below. > > On 07.11.24 10:08, David Hildenbrand wrote >> On 07.11.24 09:43, mawupeng wrote: >>> On 2024/10/31 17:47, David Hildenbrand wrote: >>>> On 30.10.24 22:32, Peter Xu wrote: >>>>> On Tue, Oct 29, 2024 at 10:03:31PM +0100, David Hildenbrand wrote: >>>>>> If track_pfn_copy() fails, we already added the dst VMA to the maple >>>>>> tree. As fork() fails, we'll cleanup the maple tree, and stumble over >>>>>> the dst VMA for which we neither performed any reservation nor copied >>>>>> any page tables. >>>>>> >>>>>> Consequently untrack_pfn() will see VM_PAT and try obtaining the >>>>>> PAT information from the page table -- which fails because the page >>>>>> table was not copied. >>>>>> >>>>>> The easiest fix would be to simply clear the VM_PAT flag of the dst VMA >>>>>> if track_pfn_copy() fails. However, the whole thing is about "simply" >>>>>> clearing the VM_PAT flag is shaky as well: if we passed track_pfn_copy() >>>>>> and performed a reservation, but copying the page tables fails, we'll >>>>>> simply clear the VM_PAT flag, not properly undoing the reservation ... >>>>>> which is also wrong. >>>>> >>>>> David, >>>>> >>>> >>>> Hi Peter, >>>> >>>>> Sorry to not have chance yet reply to your other email.. >>>>> >>>>> The only concern I have with the current fix to fork() is.. we started to >>>>> have device drivers providing fault() on PFNMAPs as vfio-pci does, then I >>>>> think it means we could potentially start to hit the same issue even >>>>> without fork(), but as long as the 1st pgtable entry of the PFNMAP range is >>>>> not mapped when the process with VM_PAT vma exit()s, or munmap() the vma. >>>> >>>> As these drivers are not using remap_pfn_range, there is no way they could currently get VM_PAT set. >>>> >>>> So what you describe is independent of the current state we are fixing here, and this fix should sort out the issues with current VM_PAT handling. >>>> >>>> It indeed is an interesting question how to handle reservations when *not* using remap_pfn_range() to cover the whole area. >>>> >>>> remap_pfn_range() handles VM_PAT automatically because it can do it: it knows that the whole range will map consecutive PFNs with the same protection, and we expect not parts of the range suddenly getting unmapped (and any driver that does that is buggy). >>>> >>>> This behavior is, however, not guaranteed to be the case when remap_pfn_range() is *not* called on the whole range. >>>> >>>> For that case (i.e., vfio-pci) I still wonder if the driver shouldn't do the reservation and leave VM_PAT alone. >>>> >>>> In the driver, we'd do the reservation once and not worry about fork() etc ... and we'd undo the reservation once the last relevant VM_PFNMAP VMA is gone or the driver let's go of the device. I assume there are already mechanisms in place to deal with that to some degree, because the driver cannot go away while any VMA still has the VM_PFNMAP mapping -- otherwise something would be seriously messed up. >>>> >>>> Long story short: let's look into not using VM_PAT for that use case. >>>> >>>> Looking at the VM_PAT issues we had over time, not making it more complicated sounds like a very reasonable thing to me :) >>> >>> Hi David, >>> >>> The VM_PAT reservation do seems complicated. It can trigger the same warning in get_pat_info if remap_p4d_range fails: >>> >>> remap_pfn_range >>> remap_pfn_range_notrack >>> remap_pfn_range_internal >>> remap_p4d_range // page allocation can failed here >>> zap_page_range_single >>> unmap_single_vma >>> untrack_pfn >>> get_pat_info >>> WARN_ON_ONCE(1); >>> >>> Any idea on this problem? >> >> In remap_pfn_range(), if remap_pfn_range_notrack() fails, we call >> untrack_pfn(), to undo the tracking. >> >> The problem is that zap_page_range_single() shouldn't do that >> untrack_pfn() call. >> >> That should be fixed by Peter's patch: >> >> https://lore.kernel.org/all/20240712144244.3090089-1-peterx@redhat.com/T/#u > > > The fix seemingly has not been applied so the issue in question still > persists. There is a long thread on that patch without an explicit > conclusion. Did the patch cause any problems or its status changed? That one still needs to be applied. Peter is currently out for a couple of weeks; I might be able to revive that in the meantime. -- Cheers, David / dhildenb