Re: [PATCH/RFC] KVM: s390: vsie/gmap: reduce gmap_rmap overhead

From: David Hildenbrand <david@redhat.com>
To: Christian Borntraeger <borntraeger@linux.ibm.com>,
	KVM <kvm@vger.kernel.org>
Cc: Janosch Frank <frankja@linux.ibm.com>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Thomas Huth <thuth@redhat.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>
Subject: Re: [PATCH/RFC] KVM: s390: vsie/gmap: reduce gmap_rmap overhead
Date: Fri, 29 Apr 2022 18:02:48 +0200	[thread overview]
Message-ID: <54927e4b-3be7-f186-747c-f14097aa2df9@redhat.com> (raw)
In-Reply-To: <20220429151526.1560-1-borntraeger@linux.ibm.com>

On 29.04.22 17:15, Christian Borntraeger wrote:
> there are cases that trigger a 2nd shadow event for the same
> vmaddr/raddr combination. (prefix changes, reboots, some known races)
> This will increase memory usages and it will result in long latencies
> when cleaning up, e.g. on shutdown. To avoid cases with a list that has
> hundreds of identical raddrs we check existing entries at insert time.
> As this measurably reduces the list length this will be faster than
> traversing the list at shutdown time.
> 
> In the long run several places will be optimized to create less entries
> and a shrinker might be necessary.
> 
> Fixes: 4be130a08420 ("s390/mm: add shadow gmap support")
> Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> ---
>  arch/s390/mm/gmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 69c08d966fda..0fc0c26a71f2 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -1185,12 +1185,19 @@ static inline void gmap_insert_rmap(struct gmap *sg, unsigned long vmaddr,
>  				    struct gmap_rmap *rmap)
>  {
>  	void __rcu **slot;
> +	struct gmap_rmap *temp;
>  
>  	BUG_ON(!gmap_is_shadow(sg));
>  	slot = radix_tree_lookup_slot(&sg->host_to_rmap, vmaddr >> PAGE_SHIFT);
>  	if (slot) {
>  		rmap->next = radix_tree_deref_slot_protected(slot,
>  							&sg->guest_table_lock);
> +		for (temp = rmap->next; temp; temp = temp->next) {
> +			if (temp->raddr == rmap->raddr) {
> +				kfree(rmap);
> +				return;
> +			}
> +		}
>  		radix_tree_replace_slot(&sg->host_to_rmap, slot, rmap);
>  	} else {
>  		rmap->next = NULL;

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb