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McKenney" Subject: Re: [PATCH v3] mm/mm_init: Don't cond_resched() in deferred_init_memmap_chunk() if called from deferred_grow_zone() Message-ID: References: <20260122184343.546627-1-longman@redhat.com> <20260122112920.2b435873a0cc5f396df5d1a7@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260122112920.2b435873a0cc5f396df5d1a7@linux-foundation.org> X-Stat-Signature: jhoh8geoc9qhqrr1nhdj1es4u5m9ccnk X-Rspamd-Queue-Id: 460E91C0002 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1769158012-292361 X-HE-Meta: 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 Z5AO8Jvb kBbe7kwiWKhluih7MUECPc5UQ2RU0BWT/FON+uIGf/MO3JOaqcg9+RJ9bNp2COMD38UX84aqcVmdSJgO2RQAxB2YwpYCqwcmImuevsjOUzHCua7B1mm7tu7lBWCZZbsXSSdMuhGo+fabGa75Wxo6/vVE5bHMLvYjlAQfLxM/kvBk8fnAX3qa5twn2h5KqZcoQFwahjfgWpYmcojAajsCC1B5glte4fXF3MDvUuYiwX8XRfjFyvzqyXet23k89LSeSN5caWq59eXMDsLFlYXrLGQGl41TdPBduEtRKaDVS2i9h0P0mcKtqG2wR57zkzUjdl3pRaQ1SoPtlUdMrp9i+cQ+mpKGfrh6b1bE5cGtnCDKK1mbBqa//lEmFNTL8a9rMa2IuEjsbiQN8rMs= 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 Thu, Jan 22, 2026 at 11:29:20AM -0800, Andrew Morton wrote: > On Thu, 22 Jan 2026 13:43:43 -0500 Waiman Long wrote: > > > Commit 3acb913c9d5b ("mm/mm_init: use deferred_init_memmap_chunk() > > in deferred_grow_zone()") made deferred_grow_zone() call > > deferred_init_memmap_chunk() within a pgdat_resize_lock() critical > > section with irqs disabled. > > > > It did check for irqs_disabled() in > > deferred_init_memmap_chunk() to avoid calling cond_resched(). For a > > PREEMPT_RT kernel build, however, spin_lock_irqsave() does not disable > > interrupt but rcu_read_lock() is called. This leads to the following > > bug report. > > > > BUG: sleeping function called from invalid context at mm/mm_init.c:2091 > > in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0 > > preempt_count: 0, expected: 0 > > > > @@ -2085,10 +2085,10 @@ deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn, > > > > spfn = chunk_end; > > > > - if (irqs_disabled()) > > - touch_nmi_watchdog(); > > - else > > + if (can_resched) > > cond_resched(); > > + else > > + touch_nmi_watchdog(); > > } > > } > > Disables the cond_resched() in some situations. Can this reintroduce > the watchdog warnings which that cond_resched() was intended to > prevent? > > The cond_resched() was added by da97f2d56bbd ("mm: call > cond_resched() from deferred_init_memmap()"). > > Pasha's 2020 patch replaced touch_nmi_watchdog() with cond_resched() to > prevent RCU stall warnings. So I think the answer to my question is > yes, going back to touch_nmi_watchdog() could reintroduce those RCU > warnings. Before 3acb913c9d5b ("mm/mm_init: use deferred_init_memmap_chunk() we had touch_nmi_watchdog() in deferred_grow_zone() and cond_resched() in the deferred_init_memmap()->deferred_init_memmap_chunk() that ran in a thread context. I thought irqs_disabled() would be enough to differentiate these cases because deferred_grow_zone() takes a spinlock, but I missed that with RT spinlock also sleeps. Using a boolean essentially restores the behaviour we had before the refactoring. -- Sincerely yours, Mike.