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[2003:d8:2f4a:8900:884a:b3af:e3c9:ec88]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-442f39ef811sm65186185e9.35.2025.05.15.06.01.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 May 2025 06:01:55 -0700 (PDT) Message-ID: <3aeb6b8e-040c-47ae-8b46-1ef66d5f11e4@redhat.com> Date: Thu, 15 May 2025 15:01:53 +0200 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] arm64: Check pxd_leaf() instead of !pxd_table() while tearing down page tables To: Ryan Roberts , Dev Jain , catalin.marinas@arm.com, will@kernel.org Cc: anshuman.khandual@arm.com, mark.rutland@arm.com, yang@os.amperecomputing.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org References: <20250515063450.86629-1-dev.jain@arm.com> <332ecda7-14c4-4dc3-aeff-26801b74ca04@redhat.com> <4904d02f-6595-4230-a321-23327596e085@arm.com> <6fe7848c-485e-4639-b65c-200ed6abe119@redhat.com> <35ef7691-7eac-4efa-838d-c504c88c042b@arm.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 15.05.25 12:07, Ryan Roberts wrote: > On 15/05/2025 09:53, David Hildenbrand wrote: >> On 15.05.25 10:47, Dev Jain wrote: >>> >>> >>> On 15/05/25 2:06 pm, David Hildenbrand wrote: >>>> On 15.05.25 10:22, Dev Jain wrote: >>>>> >>>>> >>>>> On 15/05/25 1:43 pm, David Hildenbrand wrote: >>>>>> On 15.05.25 08:34, Dev Jain wrote: >>>>>>> Commit 9c006972c3fe removes the pxd_present() checks because the caller >>>>>>> checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller >>>>>>> only >>>>>>> checks pud_present(); pud_free_pmd_page() recurses on each pmd through >>>>>>> pmd_free_pte_page(), wherein the pmd may be none. >>>>>> The commit states: "The core code already has a check for pXd_none()", >>>>>> so I assume that assumption was not true in all cases? >>>>>> >>>>>> Should that one problematic caller then check for pmd_none() instead? >>>>> >>>>>    From what I could gather of Will's commit message, my interpretation is >>>>> that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd. >>>>> These individually check for pxd_present(): >>>>> >>>>> if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) >>>>>      return 0; >>>>> >>>>> The problem is that vmap_try_huge_pud will also iterate on pte entries. >>>>> So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page >>>>> may encounter a none pmd and trigger a WARN. >>>> >>>> Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky. >>>> >>>> I assume we should either have an explicit pmd_none() check in >>>> pud_free_pmd_page() before calling pmd_free_pte_page(), or one in >>>> pmd_free_pte_page(). >>>> >>>> With your patch, we'd be calling pte_free_kernel() on a NULL pointer, >>>> which sounds wrong -- unless I am missing something important. >>> >>> Ah thanks, you seem to be right. We will be extracting table from a none >>> pmd. Perhaps we should still bail out for !pxd_present() but without the >>> warning, which the fix commit used to do. >> >> Right. We just make sure that all callers of pmd_free_pte_page() already check >> for it. >> >> I'd just do something like: > > I just reviewed the patch and had the same feedback as David. I agree with the > patch below, with some small mods... > >> >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >> index 8fcf59ba39db7..e98dd7af147d5 100644 >> --- a/arch/arm64/mm/mmu.c >> +++ b/arch/arm64/mm/mmu.c >> @@ -1274,10 +1274,8 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) >> >>         pmd = READ_ONCE(*pmdp); >> >> -       if (!pmd_table(pmd)) { >> -               VM_WARN_ON(1); >> -               return 1; >> -       } >> +       VM_WARN_ON(!pmd_present(pmd)); >> +       VM_WARN_ON(!pmd_table(pmd)); > > You don't need both of these warnings; pmd_table() is only true if the pmd is > present (well actually only if it's _valid_ which is more strict than present), > so the second one is sufficient on its own. Ah, right. > >> >>         table = pte_offset_kernel(pmdp, addr); >>         pmd_clear(pmdp); >> @@ -1305,7 +1303,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) > > Given you are removing the runtime check and early return in > pmd_free_pte_page(), I think you should modify this function to use the same > style too. BTW, the "return 1" is weird. But looking at x86, we seem to be making a private copy of the page table first, to defer freeing the page tables after the TLB flush. I wonder if there isn't a better way (e.g., clear PUDP + flush tlb, then walk over the effectively-disconnected page table). But I'm sure there is a good reason for that. > >>         next = addr; >>         end = addr + PUD_SIZE; >>         do { >> -               pmd_free_pte_page(pmdp, next); >> +               if (pmd_present(*pmdp)) > > question: I wonder if it is better to use !pmd_none() as the condition here? It > should either be none or a table at this point, so this allows the warning in > pmd_free_pte_page() to catch more error conditions. No strong opinion though. Same here. The existing callers check pmd_present(). -- Cheers, David / dhildenb