Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared memory types

From: Christian Borntraeger <borntraeger@linux.ibm.com>
To: Heiko Carstens <hca@linux.ibm.com>, Peter Xu <peterx@redhat.com>
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Subject: Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared memory types
Date: Mon, 30 May 2022 11:35:10 +0200	[thread overview]
Message-ID: <33fd4731-9765-d78b-bdc3-f8243c98e81f@linux.ibm.com> (raw)
In-Reply-To: <YpPYkzbrQmy4FjrI@osiris>

Am 29.05.22 um 22:33 schrieb Heiko Carstens:
[...]
> 
> Guess the patch below on top of your patch is what we want.
> Just for clarification: if gmap is not NULL then the process is a kvm
> process. So, depending on the workload, this optimization makes sense.
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index 4608cc962ecf..e1d40ca341b7 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -436,12 +436,11 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>   
>   	/* The fault is fully completed (including releasing mmap lock) */
>   	if (fault & VM_FAULT_COMPLETED) {
> -		/*
> -		 * Gmap will need the mmap lock again, so retake it.  TODO:
> -		 * only conditionally take the lock when CONFIG_PGSTE set.
> -		 */
> -		mmap_read_lock(mm);
> -		goto out_gmap;
> +		if (gmap) {
> +			mmap_read_lock(mm);
> +			goto out_gmap;
> +		}
> +		goto out;

Yes, that makes sense. With that

Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>