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b=uUr3y9phDo822hbAIS7QxCaW0+64MhkDdMXRtC2QnDwcw7VR21QwVcXF/3vyRk/UW k2vBpNMjbaCHvK9MjLiC6dAWWoD53uocTMzq4fln3zAbX4lPKc/eRY9sNjp/CNK3Ua KeXUzf4HdDHWIrADViwyQar8ZYOhX4ilPBmayvVlW5XVWvQTgH1tPSwY8PE7YHV0F3 6tjYmvXq5kiMqHlUOC1OzrrAbXrqB4jRAX3tsJoemB69wKewgfMlZbSJAAgRje4+sw Xt2OTYLGvj1Z77PXZS8ZwEQDQfZzkXfu4ExbROgaO3fIGZ33I8GxNT8ko5NSMfeJ4A Gb3+ovAjIXsag== Message-ID: Date: Wed, 29 Apr 2026 08:12:55 +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: <74a75b59-2e13-3985-ee99-d5521f39df2a@google.com> <20260429055447.9220-1-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: rspam12 X-Rspamd-Queue-Id: D61454000D X-Stat-Signature: haah8eruacbd33d8cjo5q5xaczbm3qb5 X-Rspam-User: X-HE-Tag: 1777443180-467839 X-HE-Meta: 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 NVzss8v6 GQKd6eroQr6KKG9zid9Et6Pnm8+qKSzjcyimuBk3qoTHa0TDWQ3cywH/npZ2WYgQYD3CV3nhM0lgxVOJ45tJfNLNOCRPrgCQ2uOJkBjQsxOW3GwY+WZ/NG2zU/J+668FGsHHuVqVKIO7RX3cHlXR6L+jBU3phCemfZRAR/ZvQQ9y8LYCIifbWhe9H8TqZM9uG6Hl++ebiXPN+NkYeYpqAi6FQz2vzDEcqdoh1ajTRxmxKytOea1EaCPOr2n8tSmZkz/9KZtgKv9l1VXSBdkuyM9nh9sPrIdaxN7LCcRYkSZXTkEMn1lvYZB6dCWrSzMZ8N/xs/NFB+4h4fOI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/29/26 07:54, Lance Yang wrote: > > On Tue, Apr 28, 2026 at 10:08:37PM -0700, Hugh Dickins wrote: >> On x86 32-bit with THP enabled, zap_huge_pmd() is seen to generate a >> "WARNING: mm/memory.c:735 at __vm_normal_page+0x6a/0x7d", from the >> VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); followed >> by "BUG: Bad rss-counter state"s, then later "BUG: Bad page state"s >> when reclaim gets to call shrink_huge_zero_folio_scan(). > > Good catch! > >> It's as if the _PAGE_SPECIAL bit never got set in the huge_zero pmd: >> and indeed, whereas pte_special() and pte_mkspecial() are subject to a >> dedicated CONFIG_ARCH_HAS_PTE_SPECIAL, pmd_special() and pmd_mkspecial() >> are subject to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP, which is never enabled >> on any 32-bit architecture. >> >> Add CONFIG_ARCH_HAS_PMD_SPECIAL? Perhaps; but I think it's better just >> to observe the huge_zero pmd in the fallback version of pmd_special(). >> >> Fixes: d80a9cb1a64a ("mm/huge_memory: add and use normal_or_softleaf_folio_pmd()") 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. 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. >> Signed-off-by: Hugh Dickins >> --- >> include/linux/mm.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index 0b776907152e..3b02ac43bcb7 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -3422,7 +3422,7 @@ static inline pte_t pte_mkspecial(pte_t pte) >> #ifndef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP >> static inline bool pmd_special(pmd_t pmd) >> { >> - return false; >> + return is_huge_zero_pmd(pmd); >> } > > Emm ... feels a bit odd to me ... Agreed. But it could be a temporary fix until we fixed up relevant architectures. > > On these configs pmd_mkspecial() is still a no-op, so pmd_special() > would no longer really mean that the PMD was made special :) > > Could we handle the huge zero PMD in vm_normal_page_pmd() instead? That adds unnecessary checks for architectures that properly implement pmd_special. pmd_special() should be fixed longterm on architectures that support THP and CONFIG_ARCH_HAS_PTE_SPECIAL. It should not be glued to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP. arch/arc/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARC_MMU_V4 arch/arm/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARM_LPAE arch/arm64/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE arch/loongarch/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE arch/mips/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES arch/powerpc/platforms/Kconfig.cputype: select HAVE_ARCH_TRANSPARENT_HUGEPAGE arch/riscv/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE if 64BIT && MMU arch/s390/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE arch/sparc/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE arch/x86/Kconfig: select HAVE_ARCH_TRANSPARENT_HUGEPAGE arch/arc/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/arm/Kconfig: select ARCH_HAS_PTE_SPECIAL if ARM_LPAE arch/arm64/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/loongarch/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/mips/Kconfig: select ARCH_HAS_PTE_SPECIAL if !(32BIT && CPU_HAS_RIXI) arch/parisc/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/powerpc/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/riscv/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/s390/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/sh/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/sparc/Kconfig: select ARCH_HAS_PTE_SPECIAL arch/x86/Kconfig: select ARCH_HAS_PTE_SPECIAL That's a bit of work given that only arm64, powerpc (64), riscv and x86 (64) properly implement pmd_special(). So I think Hugh's patch here makes sense for now. -- Cheers, David