Subject: Re: [PATCH 8/9] powerpc/mm: Wire up hpte_removebolted for powernv

From: Balbir Singh <[email-masked]>
Date: Wed, 12 Apr 2017 11:53:06 +1000

On Wed, 2017-04-12 at 03:42 +1000, Oliver O'Halloran wrote:
> From: Rashmica Gupta <[email-masked]>
> 
> Adds support for removing bolted (i.e kernel linear mapping) mappings on
> powernv. This is needed to support memory hot unplug operations which
> are required for the teardown of DAX/PMEM devices.
> 
> Cc: Rashmica Gupta <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> Signed-off-by: Oliver O'Halloran <[email-masked]>
> ---
> Could the original author of this add their S-o-b? I pulled it out of
> Rashmica's memtrace patch, but I remember someone saying Anton wrote
> it originally.
> ---
>  arch/powerpc/mm/hash_native_64.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/arch/powerpc/mm/hash_native_64.c b/arch/powerpc/mm/hash_native_64.c
> index 65bb8f33b399..9ba91d4905a4 100644
> --- a/arch/powerpc/mm/hash_native_64.c
> +++ b/arch/powerpc/mm/hash_native_64.c
> @@ -407,6 +407,36 @@ static void native_hpte_updateboltedpp(unsigned long newpp, unsigned long ea,
>  	tlbie(vpn, psize, psize, ssize, 0);
>  }
>  
> +/*
> + * Remove a bolted kernel entry. Memory hotplug uses this.
> + *
> + * No need to lock here because we should be the only user.

As long as this is after the necessary isolation and is called from
arch_remove_memory(), I think we should be fine

> + */
> +static int native_hpte_removebolted(unsigned long ea, int psize, int ssize)
> +{
> +	unsigned long vpn;
> +	unsigned long vsid;
> +	long slot;
> +	struct hash_pte *hptep;
> +
> +	vsid = get_kernel_vsid(ea, ssize);
> +	vpn = hpt_vpn(ea, vsid, ssize);
> +
> +	slot = native_hpte_find(vpn, psize, ssize);
> +	if (slot == -1)
> +		return -ENOENT;

If slot == -1, it means someone else removed the HPTE entry? Are we racing?
I suspect we should never hit this situation during hotunplug, specifically
since this is bolted.

> +
> +	hptep = htab_address + slot;
> +
> +	/* Invalidate the hpte */
> +	hptep->v = 0;

Under DEBUG or otherwise, I would add more checks like

1. was hpte_v & HPTE_V_VALID and BOLTED set? If not, we've already invalidated
that hpte and we can skip the tlbie. Since this was bolted you might be right
that it is always valid and bolted

> +
> +	/* Invalidate the TLB */
> +	tlbie(vpn, psize, psize, ssize, 0);

The API also does not clear linear_map_hash_slots[] under DEBUG_PAGEALLOC

> +	return 0;
> +}
> +
> +

Balbir Singh.