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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Jan 2026 07:43:20.5849 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 9f3625f0-d5d1-4a33-047f-08de5989ea24 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: AMS1EPF00000040.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM8PR08MB5747 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260121_234331_483626_4A575876 X-CRM114-Status: GOOD ( 17.74 ) 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 On Wed, Jan 21, 2026 at 02:57:28PM -0800, Yang Shi wrote: > > > On 1/21/26 2:20 AM, Ryan Roberts wrote: > > On 21/01/2026 08:32, Yeoreum Yun wrote: > > > > > > My concern is that if a secondary CPU can race and cause a split, that is > > > > > > unsound because we have determined that although the primary CPU supports BBML2, > > > > > > at least one of the secondary CPUs does not. So splitting a live mapping is unsafe. > > > > > > > > > > > > I just had a brief chat with Rutland, and he agrees that this _could_ be a > > > > > > problem. Basically there is a window between onlining the secondary cpus and > > > > > > entering the stop_machine() where one of those cpus _could_ end up doing > > > > > > something that causes us to split the linear map. > > > > If I remember correctly, split_kernel_leaf_mapping() does call > > > > system_supports_bbml2_noabort() before doing real split. So we basically > > > > should fall into two categories: > > > > > > > > 1. bbml2_noabort is supported on all cpus. Everything is fine. > > > > 2. bbml2_noabort is not supported on all cpus. split_kernel_leaf_mapping() > > > > just returns 0. Kernel doesn't split page table, so there won't be TLB > > > > conflict issue. But the following page prot update may see unexpected block > > > > mapping, then a   WARN  will be raised and it will return -EINVAL. So the > > > > worst case is the caller will fail (IIRC all the callers of set_memory_*() > > > > handle the failure), and we can know who is trying to change linear mapping > > > > before the linear mapping gets                    finalized. AFAICT I > > > > haven't seen such WARN yet. > > Ahh good point! So this isn't quite as terrible as I was thinking. > > Yeah. > > > > > > Thanks for the great detail :) > > > I've missed system_supports_bbml2_noabort() in split_kernel_leaf_mapping(). > > > > > > > > > I'm not immediately sure how to solve that. > > > > Do we need some synchronization mechanism? If the linear mapping is not > > > > finalized yet, split_kernel_leaf_mapping() will spin. For example, something > > > > like this off the top of my head, > > > > > > > > DEFINE_STATIC_KEY_FALSE(linear_mapping_finalized); > > > > > > > > Once the linear mapping is finalized, we can call > > > > static_branch_enable(&linear_mapping_finalized); > > > > > > > > In split_kernel_leaf_mapping(), we can just do: > > > > > > > > retry: > > > >     if (!static_branch_likely(&linear_mapping_finalized)) > > > >         goto retry; > > > > > > Yuck... But I guess it might work as long as the primary thread never does > > anything that would cause an attempt to split; otherwise we have a deadlock. > > > > > > There may be better way to handle it. But this case should be very unlikely > > > > IMHO. It sounds crazy to have such complicated kernel threads run so early. > > > > I'm not sure whether we should pay immediate attention to it or not. > > I think we need to figure out if this is actually possible. We bring up the > > secondary cpus, set system caps and finalize the linear map in smp_init(). > > That's called from kernel_init_freeable() which is called from kernel_init(), > > which is invoked as a thread pinned to the boot cpu. > > > > sched_init_smp() is called after smp_init() (i.e. after the linear map is > > finalized). I'm guessing (based on the name of sched_init_smp()) that nothing > > other than the idle thread will run on any secondaries until after > > sched_init_smp() is called? (I'd be greatful if anyone can confirm that). > > > > Rutland suggested that it's probably too early for any PM type stuff to be > > running in the idle loop, so based on all of that, perhaps this is not a problem > > after all and there is basically zero chance of a secondary cpu doing anything > > that could cause a linear map split during this window? > > > > I'm inclined to leave this as is for now. > > I agree. I don't think this would be a real problem. > > Thanks, > Yang Although partially using GFP_ATOMIC might not be an issue given that there is no contention at the moment, technically using a memory allocation API inside stop_machine() is problematic for PREEMPT_RT, and the relevant page tables should be pre-allocated. That said, taking a step back (I’m not sure why I was being so stubborn about this), since the kernel_alias area is mapped using block mappings, a simple calculation based on your dm_meminfo patch should be sufficient to determine the number of page tables that need to be pre-allocated for splitting the linear mapping, without having to walk the page tables again. So, after your dm_meminfo patch, I plan to respin this patch based on that. Am I missing anything? -- Sincerely, Yeoreum Yun