linux-kernel - Re: [PATCH] gpu: drm: v3d: use new return type vm_fault

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Date:   Tue, 3 Jul 2018 10:42:00 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     eric@...olt.net, airlied@...ux.ie, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, svptas.linux@...il.com,
        ajitn.linux@...il.com, brajeswar.linux@...il.com,
        sabyasachi.linux@...il.com
Subject: Re: [PATCH] gpu: drm: v3d: use new return type vm_fault_t

On Tue, Jul 03, 2018 at 09:40:43PM +0530, Souptick Joarder wrote:
> Use new return type vm_fault_t for fault handler. For
> now, this is just documenting that the function returns
> a VM_FAULT value rather than an errno. Once all instances
> are converted, vm_fault_t will become a distinct type.
> 
> see commit 1c8f422059ae ("mm: change return type to vm_fault_t")
> for reference.
> 
> Previously vm_insert_mixed returns err which driver
> mapped into VM_FAULT_* type. The new function
> vmf_insert_mixed will replace this inefficiency by
> returning VM_FAULT_* type.

I really think this changelog could do with more work.  Specifically for
this patch, I'd just say:

Convert v3d_gem_fault to return vm_fault_t

Instead of converting an errno into a vm_fault_t ourselves, use
vmf_insert_mixed() which returns a vm_fault_t directly.

>  	unsigned long pfn;
>  	pgoff_t pgoff;
> -	int ret;
>  
>  	/* We don't use vmf->pgoff since that has the fake offset: */
>  	pgoff = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
>  	pfn = page_to_pfn(bo->pages[pgoff]);
>  
> -	ret = vm_insert_mixed(vma, vmf->address, __pfn_to_pfn_t(pfn, PFN_DEV));
> -
> -	switch (ret) {
> -	case -EAGAIN:
> -	case 0:
> -	case -ERESTARTSYS:
> -	case -EINTR:
> -	case -EBUSY:
> -		/*
> -		 * EBUSY is ok: this just means that another thread
> -		 * already did the job.
> -		 */
> -		return VM_FAULT_NOPAGE;
> -	case -ENOMEM:
> -		return VM_FAULT_OOM;
> -	default:
> -		return VM_FAULT_SIGBUS;
> -	}
> +	return vmf_insert_mixed(vma, vmf->address,
> +		__pfn_to_pfn_t(pfn, PFN_DEV));
>  }

The line-split here is kind of ugly.  I'd do a little more extensive surgery:

-	unsigned long pfn;
+	pfn_t pfn;

- 	pfn = page_to_pfn(bo->pages[pgoff]);
+	pfn = __pfn_to_pfn_t(page_to_pfn(bo->pages[pgoff]), PFN_DEV);

+	return vmf_insert_mixed(vma, vmf->address, pfn);