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From: Demi Marie Obenour <>
To: Juergen Gross <>,
	Xen developer discussion <>,
	Andrew Morton <>
Cc: "Boris Ostrovski" <>,
	"Marek Marczykowski-Górecki" <>,,
	"Jani Nikula" <>,
	"Joonas Lahtinen" <>,
	"Rodrigo Vivi" <>,
	"Tvrtko Ursulin" <>,
	"David Airlie" <>,
	"Daniel Vetter" <>,
	"Intel Graphics Development" <>,
	"DRI Development" <>,
	"Linux Memory Management" <>,
Subject: [REGRESSION] Hang in 5.17.4+ that appears to be due to Xen
Date: Thu, 19 May 2022 12:39:40 -0400	[thread overview]
Message-ID: <YoZy6BRIkfoeY8af@itl-email> (raw)
In-Reply-To: <YoJZcUsiE3y6oul5@itl-email>

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On Mon, May 16, 2022 at 10:00:07AM -0400, Demi Marie Obenour wrote:
> On Mon, May 16, 2022 at 08:48:17AM +0200, Juergen Gross wrote:
> > On 14.05.22 17:55, Demi Marie Obenour wrote:
> > > In, a user reported
> > > that Xorg locked up when resizing a VM window.  While I do not have the
> > > same hardware the user does and thus cannot reproduce the bug, the stack
> > > trace seems to indicate a deadlock between xen_gntdev and i915.  It
> > > appears that gnttab_unmap_refs_sync() is waiting for i915 to free the
> > > pages, while i915 is waiting for the MMU notifier that called
> > > gnttab_unmap_refs_sync() to return.  Result: deadlock.
> > > 
> > > The problem appears to be that a mapped grant in PV mode will stay in
> > > the “invalidating” state until it is freed.  While MMU notifiers are
> > > allowed to sleep, it appears that they cannot wait for the page to be
> > > freed, as is happening here.  That said, I am not very familiar with
> > > this code, so my diagnosis might be incorrect.
> > 
> > All I can say for now is that your patch seems to be introducing a use after
> > free issue, as the parameters of the delayed work might get freed now before
> > the delayed work is being executed.
> I figured it was wrong, not least because I don’t think it compiles
> (invalid use of void value).  That said, the current behavior is quite
> suspicious to me.  For one, it appears that munmap() on a grant in a PV
> domain will not return until nobody else is using the page.  This is not
> what I would expect, and I can easily imagine it causing deadlocks in
> userspace.  Instead, I would expect for gntdev to automatically release
> the grant when the reference count hits zero.  This would also allow for
> the same grant to be mapped in multiple processes, and might even unlock
> DMA-BUF support.
> > I don't know why this is happening only with rather recent kernels, as the
> > last gntdev changes in this area have been made in kernel 4.13.
> > 
> > I'd suggest to look at i915, as quite some work has happened in the code
> > visible in your stack backtraces rather recently. Maybe it would be possible
> > to free the pages in i915 before calling the MMU notifier?
> While I agree that the actual problem is almost certainly in i915, the
> gntdev code does appear rather fragile.  Since so few people use i915 +
> Xen, problems with the combination generally don’t show up until some
> Qubes user makes a bug report, which isn’t great.  It would be better if
> Xen didn’t introduce requirements on other kernel code that did not hold
> when not running on Xen.
> In this case, if it is actually an invariant that one must not call MMU
> notifiers for pages that are still in use, it would be better if this
> was caught by a WARN_ON() or BUG_ON() in the core memory management
> code.  That would have found the bug instantly and deterministically on
> all platforms, whereas the current failure is nondeterministic and only
> happens under Xen.
> I also wonder if this is a bug in the core MMU notifier infrastructure.
> My reading of the mmu_interval_notifier_remove() documentation is that
> it should only wait for the specific notifier being removed to finish,
> not for all notifiers to finish.  Adding the memory management
> maintainers.

Also adding the kernel regression tracker.

#regzbot introduced v5.16..v5.17.4
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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  reply	other threads:[~2022-05-19 16:40 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-14 15:55 Demi Marie Obenour
2022-05-16  6:48 ` Juergen Gross
2022-05-16 14:00   ` Demi Marie Obenour
2022-05-19 16:39     ` Demi Marie Obenour [this message]
2022-05-19 18:17   ` Demi Marie Obenour

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