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[2003:d8:2f0c:c200:fa4a:c4ff:1b32:21ce]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4715520dd65sm3981765e9.15.2025.10.17.10.10.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Oct 2025 10:10:29 -0700 (PDT) Message-ID: Date: Fri, 17 Oct 2025 19:10:27 +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: [syzbot ci] Re: Fix stale IOTLB entries for kernel address space To: Baolu Lu , Dave Hansen , syzbot ci , akpm@linux-foundation.org, apopple@nvidia.com, bp@alien8.de, dave.hansen@linux.intel.com, iommu@lists.linux.dev, jannh@google.com, jean-philippe@linaro.org, jgg@nvidia.com, joro@8bytes.org, kevin.tian@intel.com, liam.howlett@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, luto@kernel.org, mhocko@kernel.org, mingo@redhat.com, peterz@infradead.org, robin.murphy@arm.com, rppt@kernel.org, security@kernel.org, stable@vger.kernel.org, tglx@linutronix.de, urezki@gmail.com, vasant.hegde@amd.com, vbabka@suse.cz, will@kernel.org, willy@infradead.org, x86@kernel.org, yi1.lai@intel.com Cc: syzbot@lists.linux.dev, syzkaller-bugs@googlegroups.com References: <68eeb99e.050a0220.91a22.0220.GAE@google.com> <89146527-3f41-4f1e-8511-0d06e169c09e@intel.com> <8cdb459f-f7d1-4ca0-a6a0-5c83d5092cd8@linux.intel.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 16.10.25 10:00, Baolu Lu wrote: > On 10/16/25 00:25, Dave Hansen wrote: >> Here's the part that confuses me: >> >> On 10/14/25 13:59, syzbot ci wrote: >>> page last free pid 5965 tgid 5964 stack trace: >>> reset_page_owner include/linux/page_owner.h:25 [inline] >>> free_pages_prepare mm/page_alloc.c:1394 [inline] >>> __free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2906 >>> pmd_free_pte_page+0xa1/0xc0 arch/x86/mm/pgtable.c:783 >>> vmap_try_huge_pmd mm/vmalloc.c:158 [inline] >> ... >> >> So, vmap_try_huge_pmd() did a pmd_free_pte_page(). Yet, somehow, the PMD >> stuck around so that it *could* be used after being freed. It _looks_ >> like pmd_free_pte_page() freed the page, returned 0, and made >> vmap_try_huge_pmd() return early, skipping the pmd pmd_set_huge(). >> >> But I don't know how that could possibly happen. > > The reported issue is only related to this patch: > > - [PATCH v6 3/7] x86/mm: Use 'ptdesc' when freeing PMD pages > > It appears that the pmd_ptdesc() helper can't be used directly here in > this patch. pmd_ptdesc() retrieves the page table page that the PMD > entry resides in: > > static inline struct page *pmd_pgtable_page(pmd_t *pmd) > { > unsigned long mask = ~(PTRS_PER_PMD * sizeof(pmd_t) - 1); > return virt_to_page((void *)((unsigned long) pmd & mask)); > } > > static inline struct ptdesc *pmd_ptdesc(pmd_t *pmd) > { > return page_ptdesc(pmd_pgtable_page(pmd)); > } > > while, in this patch, we need the page descriptor that a pmd entry > points to. Ah. But that's just pointing at a leaf page table, right? -- Cheers David / dhildenb