Re: [PATCH] KVM: s390: Fix access to unavailable adapter indicator pages during postcopy

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu, 21 Aug 2025 17:23:09 +0200
Thomas Huth <thuth@redhat.com> wrote:

> From: Thomas Huth <thuth@redhat.com>
> 
> When you run a KVM guest with vhost-net and migrate that guest to
> another host, and you immediately enable postcopy after starting the
> migration, there is a big chance that the network connection of the
> guest won't work anymore on the destination side after the migration.
> 
> With a debug kernel v6.16.0, there is also a call trace that looks
> like this:
> 
>  FAULT_FLAG_ALLOW_RETRY missing 881
>  CPU: 6 UID: 0 PID: 549 Comm: kworker/6:2 Kdump: loaded Not tainted 6.16.0 #56 NONE
>  Hardware name: IBM 3931 LA1 400 (LPAR)
>  Workqueue: events irqfd_inject [kvm]
>  Call Trace:
>   [<00003173cbecc634>] dump_stack_lvl+0x104/0x168
>   [<00003173cca69588>] handle_userfault+0xde8/0x1310
>   [<00003173cc756f0c>] handle_pte_fault+0x4fc/0x760
>   [<00003173cc759212>] __handle_mm_fault+0x452/0xa00
>   [<00003173cc7599ba>] handle_mm_fault+0x1fa/0x6a0
>   [<00003173cc73409a>] __get_user_pages+0x4aa/0xba0
>   [<00003173cc7349e8>] get_user_pages_remote+0x258/0x770
>   [<000031734be6f052>] get_map_page+0xe2/0x190 [kvm]
>   [<000031734be6f910>] adapter_indicators_set+0x50/0x4a0 [kvm]
>   [<000031734be7f674>] set_adapter_int+0xc4/0x170 [kvm]
>   [<000031734be2f268>] kvm_set_irq+0x228/0x3f0 [kvm]
>   [<000031734be27000>] irqfd_inject+0xd0/0x150 [kvm]
>   [<00003173cc00c9ec>] process_one_work+0x87c/0x1490
>   [<00003173cc00dda6>] worker_thread+0x7a6/0x1010
>   [<00003173cc02dc36>] kthread+0x3b6/0x710
>   [<00003173cbed2f0c>] __ret_from_fork+0xdc/0x7f0
>   [<00003173cdd737ca>] ret_from_fork+0xa/0x30
>  3 locks held by kworker/6:2/549:
>   #0: 00000000800bc958 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x7ee/0x1490
>   #1: 000030f3d527fbd0 ((work_completion)(&irqfd->inject)){+.+.}-{0:0}, at: process_one_work+0x81c/0x1490
>   #2: 00000000f99862b0 (&mm->mmap_lock){++++}-{3:3}, at: get_map_page+0xa8/0x190 [kvm]
> 
> The "FAULT_FLAG_ALLOW_RETRY missing" indicates that handle_userfaultfd()
> saw a page fault request without ALLOW_RETRY flag set, hence userfaultfd
> cannot remotely resolve it (because the caller was asking for an immediate
> resolution, aka, FAULT_FLAG_NOWAIT, while remote faults can take time).
> With that, get_map_page() failed and the irq was lost.
> 
> We should not be strictly in an atomic environment here and the worker
> should be sleepable (the call is done during an ioctl from userspace),
> so we can allow adapter_indicators_set() to just sleep waiting for the
> remote fault instead.
> 
> Link: https://issues.redhat.com/browse/RHEL-42486
> Signed-off-by: Peter Xu <peterx@redhat.com>
> [thuth: Assembled patch description and fixed some cosmetical issues]
> Signed-off-by: Thomas Huth <thuth@redhat.com>

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> ---
>  Note: Instructions for reproducing the bug can be found in the ticket here:
>  https://issues.redhat.com/browse/RHEL-42486?focusedId=26661116#comment-26661116
> 
>  arch/s390/kvm/interrupt.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> index 60c360c18690f..dcce826ae9875 100644
> --- a/arch/s390/kvm/interrupt.c
> +++ b/arch/s390/kvm/interrupt.c
> @@ -2777,12 +2777,19 @@ static unsigned long get_ind_bit(__u64 addr, unsigned long bit_nr, bool swap)
>  
>  static struct page *get_map_page(struct kvm *kvm, u64 uaddr)
>  {
> +	struct mm_struct *mm = kvm->mm;
>  	struct page *page = NULL;
> +	int locked = 1;
> +
> +	if (mmget_not_zero(mm)) {
> +		mmap_read_lock(mm);
> +		get_user_pages_remote(mm, uaddr, 1, FOLL_WRITE,
> +				      &page, &locked);
> +		if (locked)
> +			mmap_read_unlock(mm);
> +		mmput(mm);
> +	}
>  
> -	mmap_read_lock(kvm->mm);
> -	get_user_pages_remote(kvm->mm, uaddr, 1, FOLL_WRITE,
> -			      &page, NULL);
> -	mmap_read_unlock(kvm->mm);
>  	return page;
>  }
>