linux-kernel - Re: [PATCH 1/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

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Date:   Fri, 6 Apr 2018 14:25:08 +0300
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     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, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH 1/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

On 04/03/2018 12:47 PM, Daniel Vetter wrote:
> On Thu, Mar 29, 2018 at 04:19:31PM +0300, Oleksandr Andrushchenko wrote:
>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
>> +static int to_refs_grant_foreign_access(struct xen_gem_object *xen_obj)
>> +{
>> +	grant_ref_t priv_gref_head;
>> +	int ret, j, cur_ref, num_pages;
>> +	struct sg_page_iter sg_iter;
>> +
>> +	ret = gnttab_alloc_grant_references(xen_obj->num_pages,
>> +			&priv_gref_head);
>> +	if (ret < 0) {
>> +		DRM_ERROR("Cannot allocate grant references\n");
>> +		return ret;
>> +	}
>> +
>> +	j = 0;
>> +	num_pages = xen_obj->num_pages;
>> +	for_each_sg_page(xen_obj->sgt->sgl, &sg_iter, xen_obj->sgt->nents, 0) {
>> +		struct page *page;
>> +
>> +		page = sg_page_iter_page(&sg_iter);
> Quick drive-by: You can't assume that an sgt is struct page backed.
Do you mean that someone could give me sgt which never
seen sg_assign_page for its entries?
What are the other use-cases for that to happen?
> And you probably want to check this at import/attach time.
The check you mean is to make sure that when I call
page = sg_page_iter_page(&sg_iter);
I have to make sure that I get a valid page?
> -Daniel
Thank you,
Oleksandr