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b=VtXAsceR9C8okBBULjc2eiXi1M1ejfc3LFjuCJTFqBng6bWVDYvaUNmdFVcDFku5P RySSmRZ8s5lxL01Td6tJBw0ddos3n6iuj8k3py01syeZalOcRqV0Vuki/QrrnkjWsJ RmK1Kgml0DHBLQgTPccciLKQC7dE/H0yYQHdxwPPD7r36b48cbCfpK23CZvmN789FD FLle1egT0w0Igyrw1HwImr3al0TLhSifeLDG+yxBk0r91ChTzuTjH83BMTtl+/4HTR ulHkauXx2HRDodl4rjSO8zhvEP6uargrnzFdkYG4OlsI4wKQ7tf0tUG6u5nmrj8G76 37QjldtIZgolw== Message-ID: Date: Wed, 29 Apr 2026 09:14:08 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH hotfix] mm: fix pmd_special() fallback to observe huge_zero To: Lance Yang , hughd@google.com Cc: akpm@linux-foundation.org, baolin.wang@linux.alibaba.com, baohua@kernel.org, dev.jain@arm.com, liam.howlett@oracle.com, ljs@kernel.org, mhocko@suse.com, rppt@kernel.org, npache@redhat.com, zhengqi.arch@bytedance.com, ryan.roberts@arm.com, surenb@google.com, ziy@nvidia.com, linux-mm@kvack.org References: <20260429065743.67054-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 0153414000D X-Rspam-User: X-Stat-Signature: tx94ryi71kfrcdhuyfszmuyr38m459gw X-HE-Tag: 1777446853-159137 X-HE-Meta: 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 Plzdv2Rb +5b45qvCwy9PaZ6q3NzGugkcpY7R8kOJduQVXHT/mMCE6iLP+MW/x8PcJJVt1/57V6BVgDDi/MKPmevoZCgndBjmqIaR/Zg69CMXuavcghBTmEZuno4yPBhIB+iTF+DOOeeJBIxmErzuzxnU63Khk0OnPzsz6c6TCp6xcbaSZ3pj0Ia1wt8bgGK+/7d+c/flMt7Ifpopz0B+V5Vtx7keLw2p5kTlyxcumbosLL/ZdPvBHhVeCfdYd3wddO1I/15H3I3rvj5acpkXCsbLLmFEmQ6Bycc0fdBmJ619e762omn0IkzDoxLMSPkSh/g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/29/26 08:57, Lance Yang wrote: > > On Wed, Apr 29, 2026 at 08:12:55AM +0200, David Hildenbrand (Arm) wrote: >> On 4/29/26 07:54, Lance Yang wrote: >>> >>> >>> Good catch! >>> >> >> Likely it should be >> >> Fixes: d82d09e48219 ("mm/huge_memory: mark PMD mappings of the huge zero folio special") >> >> Because vm_normal_page_pmd() would return the wrong thing. > > Right. > >> But I am surprised that we didn't run into the >> >> VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); >> >> earlier. > > The history seems to be: > > 2025-09-13 d82d09e48219 ("mm/huge_memory: mark PMD mappings of the huge zero folio special") > 2025-09-13 af38538801c6 ("mm/memory: factor out common code from vm_normal_page_*()") > > After d82d09e48219, vm_normal_page_pmd() still had the explicit huge > zero check before returning the page: > > --8<--- > struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, > pmd_t pmd) > { > unsigned long pfn = pmd_pfn(pmd); > > if (unlikely(pmd_special(pmd))) > return NULL; > > if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) { > if (vma->vm_flags & VM_MIXEDMAP) { > if (!pfn_valid(pfn)) > return NULL; > goto out; > } else { > unsigned long off; > off = (addr - vma->vm_start) >> PAGE_SHIFT; > if (pfn == vma->vm_pgoff + off) > return NULL; > if (!is_cow_mapping(vma->vm_flags)) > return NULL; > } > } > > if (is_huge_zero_pfn(pfn)) > return NULL; > if (unlikely(pfn > highest_memmap_pfn)) > return NULL; > > /* > * NOTE! We still have PageReserved() pages in the page tables. > * eg. VDSO mappings can cause them to exist. > */ > out: > return pfn_to_page(pfn); > } > --- > > So even if pmd_mkspecial() was a no-op and pmd_special() stayed false, > we would still return NULL there. > > Then af38538801c6 moved the PMD path into __vm_normal_page(): > > ---8<--- > struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, > pmd_t pmd) > { > return __vm_normal_page(vma, addr, pmd_pfn(pmd), pmd_special(pmd), > pmd_val(pmd), PGTABLE_LEVEL_PMD); > } > --- > > For CONFIG_ARCH_HAS_PTE_SPECIAL=y, __vm_normal_page() only returns NULL > for the huge zero PFN if special == true. On x86 32-bit, pmd_special() > stays false, so this can now fall through to VM_WARN_ON_ONCE(): > > ---8<--- > if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) { > if (unlikely(special)) { > if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)) > return NULL; > ... > } > ... > } else { > ... > if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)) > return NULL; > } > > ... > VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); > ... > --- > > So my guess is that the warning above became possible after > af38538801c6, IIUC. Yes, I think you are right about af38538801c6. What about the following then as a temporary solution: diff --git a/mm/memory.c b/mm/memory.c index 199214f8de08..bf447c8b2f57 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -684,7 +684,9 @@ static inline struct page *__vm_normal_page(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn, bool special, unsigned long long entry, enum pgtable_level level) { - if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) { + if ((IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && level == PGTABLE_LEVEL_PTE) || + (IS_ENABLED(CONFIG_ARCH_SUPPORTS_PMD_PFNMAP) && level == PGTABLE_LEVEL_PMD) || + (IS_ENABLED(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP) && level == PGTABLE_LEVEL_PUD)) { if (unlikely(special)) { #ifdef CONFIG_FIND_NORMAL_PAGE if (vma->vm_ops && vma->vm_ops->find_normal_page) We could wrap the check in a fancy helper. -- Cheers, David