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

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Date:   Wed, 4 Dec 2019 13:32:08 +0100
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Christian König <christian.koenig@....com>,
        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

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.

Thanks,

Thomas