linux-kernel - Re: [PATCH 3/8] xen/grant-table: Allow allocating buffers suitable for DMA

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Date:   Wed, 30 May 2018 20:49:11 +0300
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, linux-media@...r.kernel.org,
        jgross@...e.com, konrad.wilk@...cle.com
Cc:     daniel.vetter@...el.com, dongwon.kim@...el.com,
        matthew.d.roper@...el.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH 3/8] xen/grant-table: Allow allocating buffers suitable
 for DMA

On 05/30/2018 06:20 PM, Boris Ostrovsky wrote:
> On 05/30/2018 02:34 AM, Oleksandr Andrushchenko wrote:
>> On 05/29/2018 10:10 PM, Boris Ostrovsky wrote:
>>> On 05/25/2018 11:33 AM, Oleksandr Andrushchenko wrote:
>>> +/**
>>> + * gnttab_dma_free_pages - free DMAable pages
>>> + * @args: arguments to the function
>>> + */
>>> +int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
>>> +{
>>> +    xen_pfn_t *frames;
>>> +    size_t size;
>>> +    int i, ret;
>>> +
>>> +    gnttab_pages_clear_private(args->nr_pages, args->pages);
>>> +
>>> +    frames = kcalloc(args->nr_pages, sizeof(*frames), GFP_KERNEL);
>>>
>>> Any way you can do it without allocating memory? One possibility is to
>>> keep allocated frames from gnttab_dma_alloc_pages(). (Not sure I like
>>> that either but it's the only thing I can think of).
>> Yes, I was also thinking about storing the allocated frames array from
>> gnttab_dma_alloc_pages(), but that seemed not to be clear enough as
>> the caller of the gnttab_dma_alloc_pages will need to store those frames
>> in some context, so we can pass them on free. But the caller doesn't
>> really
>> need the frames which might confuse, so I decided to make those
>> allocations
>> on the fly.
>> But I can still rework that to store the frames if you insist: please
>> let me know.
>
> I would prefer not to allocate anything in the release path. Yes, I
> realize that dragging frames array around is not necessary but IMO it's
> better than potentially failing an allocation during a teardown. A
> comment in the struct definition could explain the reason for having
> this field.
Then I would suggest we have it this way: current API requires that
struct page **pages are allocated from outside. So, let's allocate
the frames from outside as well. This way the caller is responsible for
both pages and frames arrays and API looks consistent.
>
>>>
>>>> +    if (!frames)
>>>> +        return -ENOMEM;
>>>> +
>>>> +    for (i = 0; i < args->nr_pages; i++)
>>>> +        frames[i] = page_to_xen_pfn(args->pages[i]);
>>> Not xen_page_to_gfn()?
>> Well, according to [1] it should be :
>>      /* XENMEM_populate_physmap requires a PFN based on Xen
>>       * granularity.
>>       */
>>      frame_list[i] = page_to_xen_pfn(page);
>
> Ah, yes. I was looking at decrease_reservation and automatically assumed
> the same parameter type.
Good, then this one is resolved
>
> -boris
>
>
Thank you,
Oleksandr