linux-kernel - Re: [syzbot] [mm?] INFO: rcu detected stall in vms_complete_munmap

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Message-ID: <1a171810-20ec-40ec-ba08-88d068243cda@lucifer.local>
Date: Fri, 25 Oct 2024 16:34:52 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: syzbot <syzbot+0ca979b86eaec7337a89@...kaller.appspotmail.com>,
        Liam.Howlett@...cle.com, akpm@...ux-foundation.org, jannh@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        vbabka@...e.cz, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [syzbot] [mm?] INFO: rcu detected stall in
 vms_complete_munmap_vmas

On Fri, Oct 25, 2024 at 11:13:33AM -0400, Alan Stern wrote:
> On Fri, Oct 25, 2024 at 12:37:36PM +0100, Lorenzo Stoakes wrote:
> > On Fri, Oct 25, 2024 at 04:23:30AM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    c6d9e43954bf Merge 6.12-rc4 into usb-next
> > > git tree:       https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git usb-testing
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=139f225f980000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=4a2bb21f91d75c65
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=0ca979b86eaec7337a89
> > > compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=179f225f980000
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/3bf4a453ec2f/disk-c6d9e439.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/e4a2db2a5d95/vmlinux-c6d9e439.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/8eb8e481b288/bzImage-c6d9e439.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+0ca979b86eaec7337a89@...kaller.appspotmail.com
> > >
> >
> > Isn't this just the same thing as [0]?
> >
> > Again I think we're just happening to hit a stall in the unmap logic rather than
> > this being an mm thing.
> >
> > We retried that one a few times and it didn't hit any mm code after the
> > first time.
> >
> > Greg mentioned there was some weirdness with the dummy usb hcd controller
> > in [1].
>
> For what it's worth, that weirdness has been fixed in 6.12-rc4 by commit
> 5189df7b8088 ("USB: gadget: dummy-hcd: Fix "task hung" problem").

Hm, but this is pointing at a commit that should have that? Strange.

I do wonder if the fact CONFIG_DEBUG_VM_MAPLE_TREE is enabled (and by the way in
the mm- stack cited) is a factor here, because that is a very very significant
slowdown anyway.

And I wonder whether it might be a good idea for syzbot to not set that flag
when doing known-time-consuming things, or at least should up the RCU stall
limits?

>
> Alan Stern