Re: [PATCH v4 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

[Paolo Bonzini]

On 10/11/22 21:58, Peter Xu wrote:
> We have had FAULT_FLAG_INTERRUPTIBLE but it was never applied to GUPs.  One
> issue with it is that not all GUP paths are able to handle signal delivers
> besides SIGKILL.
>
> That's not ideal for the GUP users who are actually able to handle these
> cases, like KVM.
>
> KVM uses GUP extensively on faulting guest pages, during which we've got
> existing infrastructures to retry a page fault at a later time.  Allowing
> the GUP to be interrupted by generic signals can make KVM related threads
> to be more responsive.  For examples:
>
>    (1) SIGUSR1: which QEMU/KVM uses to deliver an inter-process IPI,
>        e.g. when the admin issues a vm_stop QMP command, SIGUSR1 can be
>        generated to kick the vcpus out of kernel context immediately,
>
>    (2) SIGINT: which can be used with interactive hypervisor users to stop a
>        virtual machine with Ctrl-C without any delays/hangs,
>
>    (3) SIGTRAP: which grants GDB capability even during page faults that are
>        stuck for a long time.
>
> Normally hypervisor will be able to receive these signals properly, but not
> if we're stuck in a GUP for a long time for whatever reason.  It happens
> easily with a stucked postcopy migration when e.g. a network temp failure
> happens, then some vcpu threads can hang death waiting for the pages.  With
> the new FOLL_INTERRUPTIBLE, we can allow GUP users like KVM to selectively
> enable the ability to trap these signals.
>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>

If no one shouts I will queue the series for 6.2, but it would be nice 
to get an ACK from the mm folks.

> ---
>   include/linux/mm.h |  1 +
>   mm/gup.c           | 33 +++++++++++++++++++++++++++++----
>   mm/hugetlb.c       |  5 ++++-
>   3 files changed, 34 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 21f8b27bd9fd..488a9f4cce07 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2897,6 +2897,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>   #define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
>   #define FOLL_PIN	0x40000	/* pages must be released via unpin_user_page */
>   #define FOLL_FAST_ONLY	0x80000	/* gup_fast: prevent fall-back to slow gup */
> +#define FOLL_INTERRUPTIBLE  0x100000 /* allow interrupts from generic signals */
>   
>   /*
>    * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
> diff --git a/mm/gup.c b/mm/gup.c
> index 5abdaf487460..d51e7ccaef32 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -970,8 +970,17 @@ static int faultin_page(struct vm_area_struct *vma,
>   		fault_flags |= FAULT_FLAG_WRITE;
>   	if (*flags & FOLL_REMOTE)
>   		fault_flags |= FAULT_FLAG_REMOTE;
> -	if (locked)
> +	if (locked) {
>   		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
> +		/*
> +		 * FAULT_FLAG_INTERRUPTIBLE is opt-in. GUP callers must set
> +		 * FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE.
> +		 * That's because some callers may not be prepared to
> +		 * handle early exits caused by non-fatal signals.
> +		 */
> +		if (*flags & FOLL_INTERRUPTIBLE)
> +			fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
> +	}
>   	if (*flags & FOLL_NOWAIT)
>   		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
>   	if (*flags & FOLL_TRIED) {
> @@ -1380,6 +1389,22 @@ int fixup_user_fault(struct mm_struct *mm,
>   }
>   EXPORT_SYMBOL_GPL(fixup_user_fault);
>   
> +/*
> + * GUP always responds to fatal signals.  When FOLL_INTERRUPTIBLE is
> + * specified, it'll also respond to generic signals.  The caller of GUP
> + * that has FOLL_INTERRUPTIBLE should take care of the GUP interruption.
> + */
> +static bool gup_signal_pending(unsigned int flags)
> +{
> +	if (fatal_signal_pending(current))
> +		return true;
> +
> +	if (!(flags & FOLL_INTERRUPTIBLE))
> +		return false;
> +
> +	return signal_pending(current);
> +}
> +
>   /*
>    * Please note that this function, unlike __get_user_pages will not
>    * return 0 for nr_pages > 0 without FOLL_NOWAIT
> @@ -1461,11 +1486,11 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
>   		 * Repeat on the address that fired VM_FAULT_RETRY
>   		 * with both FAULT_FLAG_ALLOW_RETRY and
>   		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
> -		 * by fatal signals, so we need to check it before we
> +		 * by fatal signals of even common signals, depending on
> +		 * the caller's request. So we need to check it before we
>   		 * start trying again otherwise it can loop forever.
>   		 */
> -
> -		if (fatal_signal_pending(current)) {
> +		if (gup_signal_pending(flags)) {
>   			if (!pages_done)
>   				pages_done = -EINTR;
>   			break;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e070b8593b37..202f3ad7f35c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6206,9 +6206,12 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>   				fault_flags |= FAULT_FLAG_WRITE;
>   			else if (unshare)
>   				fault_flags |= FAULT_FLAG_UNSHARE;
> -			if (locked)
> +			if (locked) {
>   				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
>   					FAULT_FLAG_KILLABLE;
> +				if (flags & FOLL_INTERRUPTIBLE)
> +					fault_flags |= FAULT_FLAG_INTERRUPTIBLE;
> +			}
>   			if (flags & FOLL_NOWAIT)
>   				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
>   					FAULT_FLAG_RETRY_NOWAIT;