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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 21 Jan 2026 11:31:56.5974 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: cd72631e-72e9-41a1-67a7-08de58e0af1f 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: DB1PEPF000509F4.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PA4PR08MB7521 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260121_033211_051884_0E40BCD3 X-CRM114-Status: GOOD ( 15.02 ) 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 10:20:52AM +0000, 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. > > > > > 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. IIUC, since it before the sched_domain creation yet, the wake_up_new_task()'s cpu selection via select_task_rq_fair() would be always the same for prev_cpu. So I agree with you this case would be a zero chance of a scondary cpu doing anything. > > Thanks, > Ryan > > > > > Thinking about it again, I’m not sure whether > > it is acceptable to use a sleepable synchronization mechanism at this stage, > > like split_kernel_leaf_mapping() does with mutex_lock() > > (even though it may be technically possible). > > It also feels odd that this function can be called at this point in time. > > > > If this is indeed considered problematic, > > I think it would be better to simply return -EINVAL immediately > > when linear_mapping_finalized has not yet been completed. > > > > -- > > Sincerely, > > Yeoreum Yun > -- Sincerely, Yeoreum Yun