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 +X3YZdIa Iy6WDYTv2wOa6lwWE3o/0PsrQWkbn0llM39WC6NqoCD3ClgXKA8GfCTD7hYzS4Tfom3lWF3LJ3NF1XBK31X3lWI5r6rbQVer3/yrnWS1IzVtWwYTmxMI7IKv/bbxTOtFya53N 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: On Mon, Nov 10, 2025 at 12:30:06PM +0800, Qi Zheng wrote: > > > On 11/10/25 10:13 AM, Harry Yoo wrote: > > On Fri, Nov 07, 2025 at 10:32:52PM -0800, Shakeel Butt wrote: > > > On Fri, Nov 07, 2025 at 10:20:57PM +0900, Harry Yoo wrote: > > > > > > > > Although it's mentioned in the locking documentation, I'm afraid that > > > > local_lock is not the right interface to use here. Preemption will be > > > > disabled anyway (on both PREEMPT_RT and !PREEMPT_RT) when the stats are > > > > updated (in __mod_node_page_state()). > > > > > > > > Here we just want to disable IRQ only on !PREEMPT_RT (to update > > > > the stats safely). > > > > > > I don't think there is a need to disable IRQs. There are three stats > > > update functions called in that hunk. > > > > > > 1) __mod_lruvec_state > > > 2) __count_vm_events > > > 3) count_memcg_events > > > > > > count_memcg_events() can be called with IRQs. __count_vm_events can be > > > replaced with count_vm_events. > > > > Right. > > > > > For __mod_lruvec_state, the > > > __mod_node_page_state() inside needs preemption disabled. > > > > The function __mod_node_page_state() disables preemption. > > And there's a comment in __mod_zone_page_state(): > > > > > /* > > > * Accurate vmstat updates require a RMW. On !PREEMPT_RT kernels, > > > * atomicity is provided by IRQs being disabled -- either explicitly > > > * or via local_lock_irq. On PREEMPT_RT, local_lock_irq only disables > > > * CPU migrations and preemption potentially corrupts a counter so > > > * disable preemption. > > > */ > > > preempt_disable_nested(); > > > > We're relying on IRQs being disabled on !PREEMPT_RT. > > So it's possible for us to update vmstat within an interrupt context, > right? Yes, for instance when freeing memory in an interrupt context we can update vmstat and that's why we disable interrupts now. > There is also a comment above __mod_zone_page_state(): > > /* > * For use when we know that interrupts are disabled, > * or when we know that preemption is disabled and that > * particular counter cannot be updated from interrupt context. > */ Yeah we don't have to disable IRQs when we already know it's disabled. > BTW, the comment inside __mod_node_page_state() should be: > > /* See __mod_zone_page_state */ > > instead of > > /* See __mod_node_page_state */ > > Will fix it. Right :) Thanks! > > Maybe we could make it safe against re-entrant IRQ handlers by using > > read-modify-write operations? > > Isn't it because of the RMW operation that we need to use IRQ to > guarantee atomicity? Or have I misunderstood something? I meant using atomic operations instead of disabling IRQs, like, by using this_cpu_add() or cmpxchg() instead. -- Cheers, Harry / Hyeonggon