From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 97B5630C608; Mon, 19 Jan 2026 17:31:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768843909; cv=none; b=ngJb06jAEmqdAIiMkBhdUCh+DGmcOrqmN9iwRJfZWqSH/BbLpvGBedIfB//hPOl1fWYxt678QUvmzvslgfnty9qvLDqzoa27X5jXbXfDSJgSGP4sJBWIPhUsz1fwGp+TFsh8BDZzihKQjZ9+m668cbH3Ml9lfVqXyz/D29QQJ3w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768843909; c=relaxed/simple; bh=05zscD5VFLw9poBWpM3GKQq1zp3KfSkLbp1GatELe+w=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Bk7FjJ4tBRJIGEJnExqEV7AlHiHKq29y1lx7sRm+Meo7jelafCH1zgVMRML1mnRRQJmTpihuyC8mtkB7lutp58cmFRRM8r6imL1EGzkBthei698MR13MzehxdvsVvWNiPL5+/fNi1zWhXiaNuKOfFKof/OhqaHVX/bowkFD7dSg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QFRB3+Xe; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QFRB3+Xe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 57323C116C6; Mon, 19 Jan 2026 17:31:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768843909; bh=05zscD5VFLw9poBWpM3GKQq1zp3KfSkLbp1GatELe+w=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=QFRB3+XeQWq5bsyig93pQzFUB8JfoKBfpHnb/5C9iaywScMH36rSqm1TYkRlS9pIb j2rPlm1E+e6h6nF1HCipQC3QEexcTLpvJmVa/rFxHzo670Tvw7KdgrhsfMBhZrB/k2 9xmsqvibvmm2T34zYT6Qa+czaeWjbOe+MWB1QWDKCjesMpKzB6b5nco7QlO79pwCO8 KYn/G7D6FaOL9JyakbgCZD34eEOHrvxYYVM2soeoGWOSTpn6+tvGKALNEpkaK5CJad vmYH87wAmoZ7xPOOgTUvEdAOdIT0NybE/a3fTM8pmJlWo/vKTHXI81gkyuBYPGUAWA PqKOetDve4wng== Date: Mon, 19 Jan 2026 17:31:42 +0000 From: Will Deacon To: Yeoreum Yun Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, catalin.marinas@arm.com, ryan.roberts@arm.com, akpm@linux-oundation.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 Subject: Re: [PATCH v5 3/3] arm64: mmu: avoid allocating pages while installing ng-mapping for KPTI Message-ID: References: <20260105202328.2418990-1-yeoreum.yun@arm.com> <20260105202328.2418990-4-yeoreum.yun@arm.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=us-ascii Content-Disposition: inline In-Reply-To: <20260105202328.2418990-4-yeoreum.yun@arm.com> On Mon, Jan 05, 2026 at 08:23:28PM +0000, 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 | 21 ++++++++++++--------- > 1 file changed, 12 insertions(+), 9 deletions(-) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 120874a2d35b..6ea5b80ab54f 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -1360,7 +1360,7 @@ 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; > @@ -1368,10 +1368,9 @@ 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; > @@ -1382,8 +1381,6 @@ 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); > > @@ -1414,16 +1411,16 @@ 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; > /* Check whether KPTI is going to be used */ > if (!arm64_kernel_unmapped_at_el0()) > return; > @@ -1436,8 +1433,14 @@ 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"); Why are you adding this panic? Will