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From: "Thomas Hellström (VMware)" <thomas_os@shipmail.org>
To: "Christian König" <christian.koenig@amd.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org
Cc: pv-drivers@vmware.com, linux-graphics-maintainer@vmware.com,
	"Thomas Hellstrom" <thellstrom@vmware.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Michal Hocko" <mhocko@suse.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Jérôme Glisse" <jglisse@redhat.com>
Subject: Re: [PATCH 6/8] drm: Add a drm_get_unmapped_area() helper
Date: Wed, 4 Dec 2019 16:36:35 +0100
Message-ID: <14f319fd-e2ca-8f13-7bb8-4452f58c6c7e@shipmail.org> (raw)
In-Reply-To: <016a9187-1703-2d7d-0114-7fc0cbf1d121@amd.com>

On 12/4/19 3:40 PM, Christian König wrote:
> 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@vmware.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.

Yes but that adds additional complexity and considerations which made me 
think that lookup was the by far simplest choice:

- We need to modify the vma manager to care about alignments.
- Fragmentation issues. Do we want to align > 1G BOs
- For which drivers do we want to do this, how do we handle drivers that 
want to opt out in TTM mmap()?
- Non TTM drivers. Could they still reuse the same get_unmapped_area.

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

So is that an ack?


Thanks,

Thomas



>
> Regards,
> Christian.
>
>>
>> Thanks,
>>
>> Thomas
>>
>>



  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-03 13:22 [PATCH 0/8] Huge page-table entries for TTM Thomas Hellström (VMware)
2019-12-03 13:22 ` [PATCH 1/8] mm: Introduce vma_is_special_huge Thomas Hellström (VMware)
2019-12-03 13:22 ` [PATCH 2/8] mm: Split huge pages on write-notify or COW Thomas Hellström (VMware)
2019-12-03 13:22 ` [PATCH 3/8] mm: Add vmf_insert_pfn_xxx_prot() for huge page-table entries Thomas Hellström (VMware)
2019-12-03 13:22 ` [PATCH 4/8] drm/ttm, drm/vmwgfx: Support huge TTM pagefaults Thomas Hellström (VMware)
2019-12-03 13:22 ` [PATCH 5/8] drm/vmwgfx: Support huge page faults Thomas Hellström (VMware)
2019-12-03 13:22 ` [PATCH 6/8] drm: Add a drm_get_unmapped_area() helper Thomas Hellström (VMware)
2019-12-04 11:11   ` Christian König
2019-12-04 11:36     ` Thomas Hellström (VMware)
2019-12-04 12:08       ` Christian König
2019-12-04 12:32         ` Thomas Hellström (VMware)
2019-12-04 14:40           ` Christian König
2019-12-04 15:36             ` Thomas Hellström (VMware) [this message]
2019-12-03 13:22 ` [PATCH 7/8] drm/ttm: Introduce a huge page aligning TTM range manager Thomas Hellström (VMware)
2019-12-04 11:13   ` Christian König
2019-12-04 11:45     ` Thomas Hellström (VMware)
2019-12-04 12:16       ` Christian König
2019-12-04 13:18         ` Thomas Hellström (VMware)
2019-12-04 14:02           ` Christian König
2019-12-03 13:22 ` [PATCH 8/8] drm/vmwgfx: Hook up the helpers to align buffer objects Thomas Hellström (VMware)

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