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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1762262477; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0xtcF3lGaa8N2N2K4KUl4lTF49+O8kNeOdJScmNjjIs=; b=uz1bEYtO2ORgGE0p6wt3CRgbKlgG1cXsupfM7h3wlXmEbKCPhOjKfx51rOyXNcQUn78bfC 6AcsIuvy/2uAgwNSloGdAyvtw4oS+/OHRdYFLPIyFKEC9YgQ6bjfe+F3AH07wWOcXov7L/ PC80f3jGZsETMw6FxRsbiGtxTF4zBsY= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5BE1D1CE0; Tue, 4 Nov 2025 05:21:08 -0800 (PST) Received: from [10.164.18.64] (unknown [10.164.18.64]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E2E9C3F66E; Tue, 4 Nov 2025 05:21:08 -0800 (PST) Message-ID: Date: Tue, 4 Nov 2025 18:51:05 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm: Enable CONFIG_PT_RECLAIM on all architectures To: Lance Yang , zhengqi.arch@bytedance.com Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, bp@alien8.de, catalin.marinas@arm.com, dave.hansen@linux.intel.com, david@redhat.com, hannes@cmpxchg.org, hpa@zytor.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, mingo@redhat.com, ppt@kernel.org, ryan.roberts@arm.com, shakeel.butt@linux.dev, surenb@google.com, tglx@linutronix.de, vbabka@suse.cz, will@kernel.org, x86@kernel.org, Lance Yang References: <827b647d-798f-4775-bb31-ef735485c6bb@bytedance.com> <20251104131348.32332-1-ioworker0@gmail.com> Content-Language: en-US From: Dev Jain In-Reply-To: <20251104131348.32332-1-ioworker0@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 32B2C40014 X-Stat-Signature: f8j1r4hybuucuwghtb9dgy6e15fkfuk3 X-HE-Tag: 1762262477-525160 X-HE-Meta: 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 2XRVoEDV id/9IXja3Ewtkn4x6q/LPtw1cxai1YPY5GibGNAbpIM7gShtUIag6GUC7eSQVwCYwCKJBVQ9H7+aJBFkBgG+cdBH2exCTo1/P6VuFZlCdgWL/uR6vE//tvLgplgynqQ2mY5v0CEvW1WNY/XBApzYuDGPyol5NJmo/ist3fp50smOzzIAEdv5rOfpJZjECZgHoOfjc18uz2OuEaCqo6jHLPXJuU0b7qm3aHsAp+RjwfKCrgpFyBuLAmx0ekQ9RPlIyJLB4GDvN/YHz4aDBayWHui71GCbDgZsjM2OuM/MXdredY87CC4KhKdtAQQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 04/11/25 6:43 pm, Lance Yang wrote: > From: Lance Yang > > > On Tue, 4 Nov 2025 14:33:00 +0800, Qi Zheng wrote: >> >> On 11/4/25 12:02 PM, Dev Jain wrote: >>> On 03/11/25 2:37 pm, Qi Zheng wrote: >>>> Hi Dev, >>>> >>>> On 11/3/25 4:43 PM, Dev Jain wrote: >>>>> On 03/11/25 12:33 pm, Qi Zheng wrote: >>>>>> Hi Dev, >>>>>> >>>>>> On 11/3/25 2:37 PM, Dev Jain wrote: >>>>>>> The implementation of CONFIG_PT_RECLAIM is completely contained in >>>>>>> generic >>>>>>> mm code. It depends on the RCU callback which will reclaim the >>>>>>> pagetables - >>>>>>> there is nothing arch-specific about that. So, enable this config for >>>>>>> all architectures. >>>>>> Thanks for doing this! >>>>>> >>>>>> But unfortunately, not all architectures call tlb_remove_ptdesc() in >>>>>> __pte_free_tlb(). Some architectures directly call pte_free() to >>>>>> free PTE pages (without RCU). >>>>> Thanks! This was not obvious to figure out. >>>>> >>>>> Is there an arch bottleneck because of which they do this? I mean to >>>>> say, >>>>> >>>>> is something stopping us from simply redirecting __pte_free_tlb to >>>>> tlb_remove_ptdesc >>>> Some architectures have special handling in __pte_free_tlb(), and cannot >>>> simple redirect __pte_free_tlb() to tlb_remove_ptdesc(), such as m68k, >>>> powerpc, etc. >>>> >>>> For those architectures that call pte_free() in __pte_free_tlb(), it >>>> should be easy to modify them. >>>> >>>> If you're not in a rush, I can take the time to finish the above tasks. >>> Right then, I'll leave that up to you! >> OK, I will do it ASAP. > Cool! Looking forward to seeing that land ;p > > Cheers, > Lance > >>> >>>>> or pte_free_defer? >>>>> >>>>> >>>>> I am looking to enable this config at least on arm64 by default, I >>>>> believe it will be legal > Great proposal, Dev! That looks like a very useful feature. Let's make it > happen on arm64 ;) Yup, but not sure whether an arm64 enabling patch, only for that to go away when Qi implements the feature generically, is worth the trouble! > >>>>> to do this at least here. >>>> IIRC, arm64 can directly enable CONFIG_PT_RECLAIM, as it is supported >>>> at the architecture level. >>>> >>>> Thanks, >>>> Qi >>>> >>>>> >>>>>> We need to modify these architectures first, otherwise it will >>>>>> lead to UAF. This approach is feasible because Hugh provides similar >>>>>> support in pte_free_defer(). >>>>>> >>>>>> Enabling PT_RECLAIM on all architecture has always been on my >>>>>> TODO list, but it's been blocked by other things. :( >>>>>> >>>>>> Thanks, >>>>>> Qi >>>>>> >>>>>>> Signed-off-by: Dev Jain >>>>>>> --- >>>>>>>   arch/x86/Kconfig | 1 - >>>>>>>   mm/Kconfig       | 5 +---- >>>>>>>   mm/pt_reclaim.c  | 2 +- >>>>>>>   3 files changed, 2 insertions(+), 6 deletions(-) >>>>>>> >>>>>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig >>>>>>> index fa3b616af03a..5681308a5650 100644 >>>>>>> --- a/arch/x86/Kconfig >>>>>>> +++ b/arch/x86/Kconfig >>>>>>> @@ -327,7 +327,6 @@ config X86 >>>>>>>       select FUNCTION_ALIGNMENT_4B >>>>>>>       imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI >>>>>>>       select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE >>>>>>> -    select ARCH_SUPPORTS_PT_RECLAIM        if X86_64 >>>>>>>       select ARCH_SUPPORTS_SCHED_SMT        if SMP >>>>>>>       select SCHED_SMT            if SMP >>>>>>>       select ARCH_SUPPORTS_SCHED_CLUSTER    if SMP >>>>>>> diff --git a/mm/Kconfig b/mm/Kconfig >>>>>>> index 0e26f4fc8717..903c37d02555 100644 >>>>>>> --- a/mm/Kconfig >>>>>>> +++ b/mm/Kconfig >>>>>>> @@ -1355,13 +1355,10 @@ config ARCH_HAS_USER_SHADOW_STACK >>>>>>>         The architecture has hardware support for userspace shadow >>>>>>> call >>>>>>>             stacks (eg, x86 CET, arm64 GCS or RISC-V Zicfiss). >>>>>>>   -config ARCH_SUPPORTS_PT_RECLAIM >>>>>>> -    def_bool n >>>>>>> - >>>>>>>   config PT_RECLAIM >>>>>>>       bool "reclaim empty user page table pages" >>>>>>>       default y >>>>>>> -    depends on ARCH_SUPPORTS_PT_RECLAIM && MMU && SMP >>>>>>> +    depends on MMU && SMP >>>>>>>       select MMU_GATHER_RCU_TABLE_FREE >>>>>>>       help >>>>>>>         Try to reclaim empty user page table pages in paths other >>>>>>> than munmap >>>>>>> diff --git a/mm/pt_reclaim.c b/mm/pt_reclaim.c >>>>>>> index 7e9455a18aae..049e17f08c6a 100644 >>>>>>> --- a/mm/pt_reclaim.c >>>>>>> +++ b/mm/pt_reclaim.c >>>>>>> @@ -1,6 +1,6 @@ >>>>>>>   // SPDX-License-Identifier: GPL-2.0 >>>>>>>   #include >>>>>>> -#include >>>>>>> +#include >>>>>>>   #include >>>>>>>     #include "internal.h" >>