linux-kernel - Re: [PATCH 5/9] drm/xen-front: Implement handling of shared display buffers

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Date:   Fri, 23 Feb 2018 09:53:50 +0200
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, airlied@...ux.ie,
        daniel.vetter@...el.com, seanpaul@...omium.org,
        gustavo@...ovan.org, jgross@...e.com, konrad.wilk@...cle.com
Cc:     Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH 5/9] drm/xen-front: Implement handling of shared display
 buffers

On 02/23/2018 02:25 AM, Boris Ostrovsky wrote:
> On 02/21/2018 03:03 AM, Oleksandr Andrushchenko wrote:
>>   
>>   static int __init xen_drv_init(void)
>>   {
>> +	/* At the moment we only support case with XEN_PAGE_SIZE == PAGE_SIZE */
>> +	BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
>
> Why BUILD_BUG_ON? This should simply not load if page sizes are different.
>
>   
This is a compile time check, so if kernel/Xen is configured
to use page size combination which is not supported by the
driver it will fail during compilation. This seems correct to me,
because you shouldn't even try to load the driver which
cannot handle different page sizes to not make any harm.
>
>
>
>> +	ret = gnttab_map_refs(map_ops, NULL, buf->pages, buf->num_pages);
>> +	BUG_ON(ret);
>
> We should try not to BUG*(). There are a few in this patch (and possibly
> others) that I think can be avoided.
>
I will rework BUG_* for map/unmap code to handle errors,
but will still leave
     /* either pages or sgt, not both */
     BUG_ON(cfg->pages && cfg->sgt);
which is a real driver bug and must not happen
>
>
>
>> +
>> +static int alloc_storage(struct xen_drm_front_shbuf *buf)
>> +{
>> +	if (buf->sgt) {
>> +		buf->pages = kvmalloc_array(buf->num_pages,
>> +				sizeof(struct page *), GFP_KERNEL);
>> +		if (!buf->pages)
>> +			return -ENOMEM;
>> +
>> +		if (drm_prime_sg_to_page_addr_arrays(buf->sgt, buf->pages,
>> +				NULL, buf->num_pages) < 0)
>> +			return -EINVAL;
>> +	}
>> +
>> +	buf->grefs = kcalloc(buf->num_grefs, sizeof(*buf->grefs), GFP_KERNEL);
>> +	if (!buf->grefs)
>> +		return -ENOMEM;
>> +
>> +	buf->directory = kcalloc(get_num_pages_dir(buf), PAGE_SIZE, GFP_KERNEL);
>> +	if (!buf->directory)
>> +		return -ENOMEM;
> You need to clean up on errors.
this is called in xen_drm_front_shbuf_alloc and is properly cleaned
on failure, e.g.:
     ret = alloc_storage(buf);
     if (ret)
         goto fail;
[...]
fail:
     xen_drm_front_shbuf_free(buf);

> -boris
>
>> +
>> +	return 0;
>> +}