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Howlett" To: Josh Poimboeuf , Harry Yoo , Lorenzo Stoakes , syzbot , akpm@linux-foundation.org, jannh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, pfalcato@suse.de, syzkaller-bugs@googlegroups.com, vbabka@suse.cz, Sebastian Andrzej Siewior , peterz@infradead.org Subject: Re: [syzbot] [mm?] INFO: rcu detected stall in sys_munmap (2) Message-ID: Mail-Followup-To: "Liam R. 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version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: * Liam R. Howlett [250827 21:58]: > * Josh Poimboeuf [250827 20:29]: > > On Fri, Aug 22, 2025 at 10:55:10PM +0900, Harry Yoo wrote: > > > On Fri, Aug 22, 2025 at 01:08:02PM +0100, Lorenzo Stoakes wrote: > > > > +cc Sebastian for RCU ORC change... > > > > > > > > +cc Harry for slab side. > > > > > > +cc Josh and Peter for stack unwinding stuff. > > > > > > > Pinging Jann for the CONFIG_SLUB_RCU_DEBUG element. > > > > > > > > Jann - could this possibly be related to CONFIG_SLUB_RCU_DEBUG? As it seems to > > > > the stack is within KASAN, but no KASAN report so maybe it's KASAN itself that's > > > > having an issue? > > > > > > > > Though I'm thinking maybe it's the orc unwinder itself that could be problematic > > > > here (yet invoked by CONFIG_SLUB_RCU_DEBUG though)... and yeah kinda suspcious > > > > because: > > > > > > > > - We have two threads freeing VMAs using SLAB_TYPESAFE_BY_RCU > > > > - CONFIG_SLUB_RCU_DEBUG means that we use KASAN to save an aux stack, which > > > > makes us do an unwind via ORC, which then takes an RCU read lock on > > > > unwind_next_frame(), and both are doing this unwinding at the time of report. > > > > - ??? > > > > - Somehow things get locked up? > > > > > > > > I'm not an RCU expert (clearly :) so I'm not sure exactly how this could result > > > > in a stall, but it's suspicious. > > > > > > Can this be because of misleading ORC data or logical error in ORC unwinder > > > that makes it fall into an infinite loop (unwind_done() never returning > > > true in arch_stack_walk())? > > > > > > ...because the reported line number reported doesn't really make sense > > > as a cause of stalls. > > > > There shouldn't be any way for ORC to hit an infinite loop. Worst case > > it would stop after the caller's buffer fills up. ORC has always been > > solid, and the RCU usage looks fine to me. I tend to doubt ORC is at > > fault here. > > > > Maybe some interaction higher up the stack is causing things to run in a > > tight loop. > > > > All those debugging options (e.g., DEBUG_VM_MAPLE_TREE, LOCKDEP, KASAN, > > SLUB_RCU_DEBUG...) could be a factor in slowing things down to a crawl. > > DEBUG_VM_MAPLE_TREE is super heavy, but that comes from validate_mm() > which would be the last thing to happen before returning, usually. > > I mean surely that would show up in the logs. > > Okay it's in the second log on the dashboard.. > > Yeah, I think it's debug options eventually causing failure. Apparently > there's a reproducer for syz now but without the validate_mm(). I don't think it's the debugging options as removing the validate_mm() did not help. We may want to wait for a c reproducer.