linux-kernel - Re: [PATCH] ocxl: Allow contexts to be attached with a NULL mm

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Date:   Tue, 18 Jun 2019 11:50:59 +1000
From:   Andrew Donnellan <ajd@...ux.ibm.com>
To:     "Alastair D'Silva" <alastair@....ibm.com>, alastair@...ilva.org
Cc:     Frederic Barrat <fbarrat@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ocxl: Allow contexts to be attached with a NULL mm

On 17/6/19 2:41 pm, Alastair D'Silva wrote:
> From: Alastair D'Silva <alastair@...ilva.org>
> 
> If an OpenCAPI context is to be used directly by a kernel driver, there
> may not be a suitable mm to use.
> 
> The patch makes the mm parameter to ocxl_context_attach optional.
> 
> Signed-off-by: Alastair D'Silva <alastair@...ilva.org>

The one issue I can see here is that using mm == NULL bypasses our 
method of enabling/disabling global TLBIs in mm_context_add_copro().

Discussing this privately with Alastair and Fred - this should be fine, 
but perhaps we should document that.

> ---
>   drivers/misc/ocxl/context.c |  9 ++++++---
>   drivers/misc/ocxl/link.c    | 12 ++++++++----
>   2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/misc/ocxl/context.c b/drivers/misc/ocxl/context.c
> index bab9c9364184..994563a078eb 100644
> --- a/drivers/misc/ocxl/context.c
> +++ b/drivers/misc/ocxl/context.c
> @@ -69,6 +69,7 @@ static void xsl_fault_error(void *data, u64 addr, u64 dsisr)
>   int ocxl_context_attach(struct ocxl_context *ctx, u64 amr, struct mm_struct *mm)
>   {
>   	int rc;
> +	unsigned long pidr = 0;
>   
>   	// Locks both status & tidr
>   	mutex_lock(&ctx->status_mutex);
> @@ -77,9 +78,11 @@ int ocxl_context_attach(struct ocxl_context *ctx, u64 amr, struct mm_struct *mm)
>   		goto out;
>   	}
>   
> -	rc = ocxl_link_add_pe(ctx->afu->fn->link, ctx->pasid,
> -			mm->context.id, ctx->tidr, amr, mm,
> -			xsl_fault_error, ctx);
> +	if (mm)
> +		pidr = mm->context.id;
> +
> +	rc = ocxl_link_add_pe(ctx->afu->fn->link, ctx->pasid, pidr, ctx->tidr,
> +			      amr, mm, xsl_fault_error, ctx);
>   	if (rc)
>   		goto out;
>   
> diff --git a/drivers/misc/ocxl/link.c b/drivers/misc/ocxl/link.c
> index cce5b0d64505..43542f124807 100644
> --- a/drivers/misc/ocxl/link.c
> +++ b/drivers/misc/ocxl/link.c
> @@ -523,7 +523,8 @@ int ocxl_link_add_pe(void *link_handle, int pasid, u32 pidr, u32 tidr,
>   	pe->amr = cpu_to_be64(amr);
>   	pe->software_state = cpu_to_be32(SPA_PE_VALID);
>   
> -	mm_context_add_copro(mm);
> +	if (mm)
> +		mm_context_add_copro(mm);
>   	/*
>   	 * Barrier is to make sure PE is visible in the SPA before it
>   	 * is used by the device. It also helps with the global TLBI
> @@ -546,7 +547,8 @@ int ocxl_link_add_pe(void *link_handle, int pasid, u32 pidr, u32 tidr,
>   	 * have a reference on mm_users. Incrementing mm_count solves
>   	 * the problem.
>   	 */
> -	mmgrab(mm);
> +	if (mm)
> +		mmgrab(mm);
>   	trace_ocxl_context_add(current->pid, spa->spa_mem, pasid, pidr, tidr);
>   unlock:
>   	mutex_unlock(&spa->spa_lock);
> @@ -652,8 +654,10 @@ int ocxl_link_remove_pe(void *link_handle, int pasid)
>   	if (!pe_data) {
>   		WARN(1, "Couldn't find pe data when removing PE\n");
>   	} else {
> -		mm_context_remove_copro(pe_data->mm);
> -		mmdrop(pe_data->mm);
> +		if (pe_data->mm) {
> +			mm_context_remove_copro(pe_data->mm);
> +			mmdrop(pe_data->mm);
> +		}
>   		kfree_rcu(pe_data, rcu);
>   	}
>   unlock:
> 

-- 
Andrew Donnellan              OzLabs, ADL Canberra
ajd@...ux.ibm.com             IBM Australia Limited