linux-kernel - Re: [PATCH 6/8] drm: Add a drm_get_unmapped

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Date:   Wed, 4 Dec 2019 15:40:09 +0100
From:   Christian König <christian.koenig@....com>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Cc:     pv-drivers@...are.com, linux-graphics-maintainer@...are.com,
        Thomas Hellstrom <thellstrom@...are.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH 6/8] drm: Add a drm_get_unmapped_area() helper

Am 04.12.19 um 13:32 schrieb Thomas Hellström (VMware):
> On 12/4/19 1:08 PM, Christian König wrote:
>> Am 04.12.19 um 12:36 schrieb Thomas Hellström (VMware):
>>> On 12/4/19 12:11 PM, Christian König wrote:
>>>> Am 03.12.19 um 14:22 schrieb Thomas Hellström (VMware):
>>>>> From: Thomas Hellstrom <thellstrom@...are.com>
>>>>>
>>>>> This helper is used to align user-space buffer object addresses to
>>>>> huge page boundaries, minimizing the chance of alignment mismatch
>>>>> between user-space addresses and physical addresses.
>>>>
>>>> Mhm, I'm wondering if that is really such a good idea.
>>>
>>> Could you elaborate? What drawbacks do you see?
>>
>> Main problem for me seems to be that I don't fully understand what 
>> the get_unmapped_area callback is doing.
>
> It makes sure that, if there is a chance that we could use huge 
> page-table entries, virtual address huge page boundaries are perfectly 
> aligned to physical address huge page boundaries, which is if not a 
> CPU hardware requirement, at least a kernel requirement currently.
>
>
>>
>> For example why do we need to use drm_vma_offset_lookup_locked() to 
>> adjust the pgoff?
>>
>> The mapped offset should be completely irrelevant for finding some 
>> piece of userspace address space or am I totally off here?
>
>
> Because the unmodified pgoff assumes that physical address boundaries 
> are perfectly aligned with file offset boundaries, which is typical 
> for all other subsystems.
>
> That's not true for TTM, however, where a buffer object start physical 
> address may be huge page aligned, but the file offset is always page 
> aligned. We could of course change that to align also file offsets to 
> huge page size boundaries, but with the above adjustment, that's not 
> needed. I opted for the adjustment.

I would opt for aligning the file offsets instead.

Now that you explained it that the rest of the kernel enforces this 
actually makes sense.

Regards,
Christian.

>
> Thanks,
>
> Thomas
>
>