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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Jan 2026 21:32:49.3620 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 9c6c1d1e-2b2c-461b-4e2a-08de57a24b65 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00025F9B.EURPRD83.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PAWPR08MB10996 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260119_133257_324379_C82112FE X-CRM114-Status: GOOD ( 21.14 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Will, > 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? Because original code call the panic() when it fails to create mapping of kpti_ng_temp_pgd and I think allocation is also part of creating kpti_ng_temp_pgd too. So, I added this panic() when allocation is failed for kpti_ng_temp_pgd. -- Sincerely, Yeoreum Yun