From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 647592DFF04 for ; Fri, 2 Jan 2026 11:10:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767352231; cv=none; b=tFAvxrkipXC9teH7efv6kwa5baNmP8uAun7brSMXq/UFGbhkX0uTqzPOA3xNnYQCJLVQ1iSJQfuSjJQcu4smsszEz0GTUi9/yFZSMA8t2r4AToAkDYKrYMbkVJ7+qjBMNzkoFKSw8skuzfMv1V/SPlZfkrXaTfD0ofmxIojkUvA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767352231; c=relaxed/simple; bh=DITRn2hJUkiQP39zpfAGL88UD70iMXgdzf3rtq5ZHfo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PXc6DWKXWnYCnd/KHdmfBKg1jsWnXgugx/EYvhCWxrhLmI0N2se/A6oArU+PV0yOy9OjuehEz/JS0Swi8ulo6dW/ityKhisLUywt+dgSfFye6NxKFPN9mtN658YLk/C8cBLD3967cPoS3Y2UHExR8fEEuJE8HZJ+zg1RDgPnwZU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 38E66497; Fri, 2 Jan 2026 03:10:20 -0800 (PST) Received: from [10.57.94.221] (unknown [10.57.94.221]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 88A0E3F63F; Fri, 2 Jan 2026 03:10:23 -0800 (PST) Message-ID: Date: Fri, 2 Jan 2026 11:10:21 +0000 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: [PATCH v3 2/2] arm64: mmu: avoid allocating pages while installing ng-mapping for KPTI Content-Language: en-GB To: Yeoreum Yun , catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, david@kernel.org, kevin.brodsky@arm.com, quic_zhenhuah@quicinc.com, dev.jain@arm.com, yang@os.amperecomputing.com, chaitanyas.prakash@arm.com, bigeasy@linutronix.de, clrkwllms@kernel.org, rostedt@goodmis.org, lorenzo.stoakes@oracle.com, ardb@kernel.org, jackmanb@google.com, vbabka@suse.cz, mhocko@suse.com Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev References: <20251218194750.395301-1-yeoreum.yun@arm.com> <20251218194750.395301-3-yeoreum.yun@arm.com> From: Ryan Roberts In-Reply-To: <20251218194750.395301-3-yeoreum.yun@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 18/12/2025 19:47, Yeoreum Yun wrote: > The current __kpti_install_ng_mappings() allocates a temporary PGD > while installing the NG mapping for KPTI under stop_machine(), > using GFP_ATOMIC. > > This is fine in the non-PREEMPT_RT case. However, it becomes a problem > under PREEMPT_RT because generic memory allocation/free APIs > (e.g., pgtable_alloc(), __get_free_pages(), etc.) cannot be invoked > in a non-preemptible context, except for the *_nolock() variants. > These generic allocators may sleep due to their use of spin_lock(). > > In other words, calling __get_free_pages(), even with GFP_ATOMIC, > is not allowed in __kpti_install_ng_mappings(), which is executed by > the stopper thread where preemption is disabled under PREEMPT_RT. > > To address this, preallocate the page needed for the temporary PGD > before invoking __kpti_install_ng_mappings() via stop_machine(). > > Fixes: 47546a1912fc ("arm64: mm: install KPTI nG mappings with MMU enabled") > Signed-off-by: Yeoreum Yun > Reviewed-by: Ryan Roberts > --- > arch/arm64/mm/mmu.c | 22 +++++++++++++--------- > 1 file changed, 13 insertions(+), 9 deletions(-) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 96a9fa505e71..9ad9612728e6 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -1369,7 +1369,7 @@ static phys_addr_t __init kpti_ng_pgd_alloc(enum pgtable_type type) > return kpti_ng_temp_alloc; > } > > -static int __init __kpti_install_ng_mappings(void *__unused) > +static int __init __kpti_install_ng_mappings(void *data) > { > typedef void (kpti_remap_fn)(int, int, phys_addr_t, unsigned long); > extern kpti_remap_fn idmap_kpti_install_ng_mappings; > @@ -1377,10 +1377,9 @@ static int __init __kpti_install_ng_mappings(void *__unused) > > int cpu = smp_processor_id(); > int levels = CONFIG_PGTABLE_LEVELS; > - int order = order_base_2(levels); > u64 kpti_ng_temp_pgd_pa = 0; > pgd_t *kpti_ng_temp_pgd; > - u64 alloc = 0; > + u64 alloc = *(u64 *)data; > > if (levels == 5 && !pgtable_l5_enabled()) > levels = 4; > @@ -1391,8 +1390,6 @@ static int __init __kpti_install_ng_mappings(void *__unused) > > if (!cpu) { > int ret; > - > - alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order); > kpti_ng_temp_pgd = (pgd_t *)(alloc + (levels - 1) * PAGE_SIZE); > kpti_ng_temp_alloc = kpti_ng_temp_pgd_pa = __pa(kpti_ng_temp_pgd); > > @@ -1423,16 +1420,17 @@ static int __init __kpti_install_ng_mappings(void *__unused) > remap_fn(cpu, num_online_cpus(), kpti_ng_temp_pgd_pa, KPTI_NG_TEMP_VA); > cpu_uninstall_idmap(); > > - if (!cpu) { > - free_pages(alloc, order); > + if (!cpu) > arm64_use_ng_mappings = true; > - } > > return 0; > } > > void __init kpti_install_ng_mappings(void) > { > + int order = order_base_2(CONFIG_PGTABLE_LEVELS); > + u64 alloc; > + nit: Restore the blank line between the variable definitioins and the logic. But you already have my R-b :) > /* Check whether KPTI is going to be used */ > if (!arm64_kernel_unmapped_at_el0()) > return; > @@ -1445,8 +1443,14 @@ void __init kpti_install_ng_mappings(void) > if (arm64_use_ng_mappings) > return; > > + alloc = __get_free_pages(GFP_KERNEL | __GFP_ZERO, order); > + if (!alloc) > + panic("Failed to alloc page tables\n"); > + > init_idmap_kpti_bbml2_flag(); > - stop_machine(__kpti_install_ng_mappings, NULL, cpu_online_mask); > + stop_machine(__kpti_install_ng_mappings, &alloc, cpu_online_mask); > + > + free_pages(alloc, order); > } > > static pgprot_t __init kernel_exec_prot(void)