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Tue, 04 Nov 2025 05:15:32 -0800 (PST) From: Lance Yang To: zhengqi.arch@bytedance.com, dev.jain@arm.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 Subject: Re: [RFC PATCH] mm: Enable CONFIG_PT_RECLAIM on all architectures Date: Tue, 4 Nov 2025 21:15:21 +0800 Message-ID: <20251104131522.33079-1-ioworker0@gmail.com> X-Mailer: git-send-email 2.49.0 In-Reply-To: <827b647d-798f-4775-bb31-ef735485c6bb@bytedance.com> References: <827b647d-798f-4775-bb31-ef735485c6bb@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 > > > > > > >> > >>> > >>> 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 ;) > >>> > >>> 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" > >>>> > >> > >