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X-CSE-ConnectionGUID: HFCLSrpqSDOjuqHpgyvNjQ== X-CSE-MsgGUID: C55UPbSpTEuGWg1E6YvV0g== X-IronPort-AV: E=McAfee;i="6800,10657,11799"; a="92206949" X-IronPort-AV: E=Sophos;i="6.24,171,1774335600"; d="scan'208";a="92206949" Received: from orviesa010.jf.intel.com ([10.64.159.150]) by orvoesa104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 May 2026 08:55:43 -0700 X-CSE-ConnectionGUID: 7m+ylDAET0a9fVsYihiZMw== X-CSE-MsgGUID: UAoPuixzRUyLGu7nrdfhxg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.24,171,1774335600"; d="scan'208";a="241441695" Received: from sghuge-mobl2.amr.corp.intel.com (HELO [10.125.111.27]) ([10.125.111.27]) by orviesa010-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 May 2026 08:55:41 -0700 Message-ID: <4e0696ee-895c-486d-aacc-71146f0e9047@intel.com> Date: Wed, 27 May 2026 08:55:08 -0700 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: [RFC PATCH 1/3] mm: make persistent huge zero folio read-only To: Xueyuan chen , akpm@linux-foundation.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, x86@kernel.org, catalin.marinas@arm.com, will@kernel.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, david@kernel.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, ryan.roberts@arm.com, dev.jain@arm.com, lance.yang@linux.dev, yang@os.amperecomputing.com, jannh@google.com References: <20260527035607.14919-1-xueyuan.chen21@gmail.com> <20260527035607.14919-2-xueyuan.chen21@gmail.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/26/26 20:56, Xueyuan chen wrote:> +#ifdef CONFIG_READONLY_HUGE_ZERO_FOLIO > +bool __init arch_make_huge_zero_folio_readonly(struct folio *folio); > +#endif All of the #ifdeffery needs to die, IMNHO. This function is also a bad idea. There is nothing "huge zero" specific about it. It takes any old folio and tries to make it read only. Just make it: bool __init arch_make_folio_readonly(struct folio *folio) Make it a weak symbol with a stub in some mm/foo.c file, and then the architectures can override it if they want. > static inline struct folio *get_persistent_huge_zero_folio(void) > { > if (!IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO)) > diff --git a/mm/Kconfig b/mm/Kconfig > index 776b67c66e82..f31200816646 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -787,6 +787,23 @@ config PERSISTENT_HUGE_ZERO_FOLIO > Say Y if your system has lots of memory. Say N if you are > memory constrained. > > +config ARCH_HAS_READONLY_HUGE_ZERO_FOLIO > + bool > + > +config READONLY_HUGE_ZERO_FOLIO > + bool "Map the huge zero folio read-only in the direct map" > + depends on PERSISTENT_HUGE_ZERO_FOLIO > + depends on ARCH_HAS_READONLY_HUGE_ZERO_FOLIO > + help > + The persistent huge zero folio is shared globally, and nothing > + should ever change its contents after initialization. > + > + When supported, mark the folio read-only in the direct map so such > + writes trigger a fault instead of silently corrupting the zero contents. > + > + If the permission change is not supported, the kernel keeps using > + the writable persistent huge zero folio. I vote for no Kconfig options here. Why? This adds "security" with _basically_ no extra runtime cost. The runtime cost is, what, usually one kernel TLB invalidation during boot? If you desperately need one, you can add it without a prompt so folks need to edit .config files to change the values. > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index bf9b480bb3b0..c568755dd58e 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -75,7 +75,11 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, > static bool split_underused_thp = true; > > static atomic_t huge_zero_refcount; > +#ifdef CONFIG_READONLY_HUGE_ZERO_FOLIO > +struct folio *huge_zero_folio __ro_after_init; > +#else > struct folio *huge_zero_folio __read_mostly; > +#endif #ifdefs bad. Bad. Bad. Bad. ;) Seriously, just pick one. Lowest common denominator is more important that being precisely correct all the time. > unsigned long huge_zero_pfn __read_mostly = ~0UL; > unsigned long huge_anon_orders_always __read_mostly; > unsigned long huge_anon_orders_madvise __read_mostly; > @@ -305,6 +309,18 @@ static unsigned long shrink_huge_zero_folio_scan(struct shrinker *shrink, > return 0; > } > > +#ifdef CONFIG_READONLY_HUGE_ZERO_FOLIO > +static bool __init make_huge_zero_folio_readonly(void) > +{ > + return arch_make_huge_zero_folio_readonly(READ_ONCE(huge_zero_folio)); > +} > +#else > +static bool __init make_huge_zero_folio_readonly(void) > +{ > + return false; > +} > +#endif > + > static struct shrinker *huge_zero_folio_shrinker; > > #ifdef CONFIG_SYSFS > @@ -965,8 +981,15 @@ static int __init thp_shrinker_init(void) > * that get_huge_zero_folio() will most likely not fail as > * thp_shrinker_init() is invoked early on during boot. > */ > - if (!get_huge_zero_folio()) > + if (!get_huge_zero_folio()) { > pr_warn("Allocating persistent huge zero folio failed\n"); > + return 0; > + } > + > + if (IS_ENABLED(CONFIG_READONLY_HUGE_ZERO_FOLIO) && > + !make_huge_zero_folio_readonly()) > + pr_warn("Making persistent huge zero folio read-only failed\n"); > + > return 0; The IS_ENABLED() doesn't do anything from what I can tell. This is all in one .c file. The compiler can see that make_huge_zero_folio_readonly() is always false. It should optimize out the pr_warn() for you.