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

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Date:   Wed, 30 May 2018 15:25:33 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Oleksandr Andrushchenko <andr2000@...il.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 01:49 PM, Oleksandr Andrushchenko wrote:
> 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.


Yes, that works too.

-boris