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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 16 Dec 2025 11:04:21.6465 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 1944617f-fa00-4a4d-8d55-08de3c92ddcb 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: DU2PEPF00028D09.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS4PR08MB8191 X-Rspamd-Queue-Id: 55DF540010 X-Stat-Signature: xb1sbdo8tyom9gie1hst9jiqig1ezg39 X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1765883070-971520 X-HE-Meta: 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 9gPqaPPm 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Brendan, > On Mon Dec 15, 2025 at 10:06 AM UTC, Yeoreum Yun wrote: > [snip] > >> Overall I am feeling a bit uncomfortable about this use of _nolock, but > >> I am also feeling pretty ignorant about PREEMPT_RT and also about this > >> arm64 code, so I am hesitant to suggest alternatives, I hope someone > >> else can offer some input here... > > > > I understand. However, as I mentioned earlier, > > my main intention was to hear opinions specifically about memory contention. > > > > That said, if there is no memory contention, > > I don’t think using the _nolock API is necessarily a bad approach. > > > > In fact, I believe a bigger issue is that, under PREEMPT_RT, > > code that uses the regular memory allocation APIs may give users the false impression > > that those APIs are “safe to use,” even though they are not. > > Yeah, I share this concern. I would bet I have written code that's > broken under PREEMPT_RT (luckily only in Google's kernel fork). The > comment for GFP_ATOMIC says: > > * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > * watermark is applied to allow access to "atomic reserves". > * The current implementation doesn't support NMI and few other strict > * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT. > > It kinda sounds like it's supposed to be OK to use GFP_ATOMIC in a > normal preempt_disable() context. So do you know exactly why it's > invalid to use it in this stop_machine() context here? Maybe we need to > update this comment. In non-PREEMPT_RT configurations, this is fine to use. However, in PREEMPT_RT, it should not be used because spin_lock becomes a sleepable lock backed by an rt-mutex. >From Documentation/locking/locktypes.rst: The fact that PREEMPT_RT changes the lock category of spinlock_t and rwlock_t from spinning to sleeping. As you know, all locks related to memory allocation (e.g., zone_lock, PCP locks, etc.) use spin_lock, which becomes sleepable under PREEMPT_RT. The callback of stop_machine() is executed in a preemption-disabled context (see cpu_stopper_thread()). In this context, if it fails to acquire a spinlock during memory allocation, the task would be able to go to sleep while preemption is disabled, which is an obviously problematic situation. > Or maybe actually we need to fix the allocator > so that GFP_ATOMIC allocs are safe in this context? I don’t think so, because GFP_ATOMIC can still be used by IRQ threads in PREEMPT_RT. (If an IRQ handler is non-threaded due to IRQF_NO_THREAD, then it cannot use it.) Although the root cause appears to be that the memory allocator uses spin_lock(), I believe the real issue is where such allocations are allowed to be used. If we changed the lock to raw_spin_lock(), this would introduce significant latency during memory allocation. So I believe this is why the memory allocator continues to use spin_lock(). IOW, what I really want to ask is whether general memory allocation/free operations are permissible in a stop_machine() context (I think nolock() can be allowable only). Thanks -- Sincerely, Yeoreum Yun