Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

["Thomas Hellström (Intel)" <thomas_os@shipmail.org>]

On 3/24/21 1:24 PM, Jason Gunthorpe wrote:
> On Wed, Mar 24, 2021 at 10:56:43AM +0100, Daniel Vetter wrote:
>> On Tue, Mar 23, 2021 at 06:06:53PM +0100, Thomas Hellström (Intel) wrote:
>>> On 3/23/21 5:37 PM, Jason Gunthorpe wrote:
>>>> On Tue, Mar 23, 2021 at 05:34:51PM +0100, Thomas Hellström (Intel) wrote:
>>>>
>>>>>>> @@ -210,6 +211,20 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
>>>>>>>     	if ((pfn & (fault_page_size - 1)) != 0)
>>>>>>>     		goto out_fallback;
>>>>>>> +	/*
>>>>>>> +	 * Huge entries must be special, that is marking them as devmap
>>>>>>> +	 * with no backing device map range. If there is a backing
>>>>>>> +	 * range, Don't insert a huge entry.
>>>>>>> +	 * If this check turns out to be too much of a performance hit,
>>>>>>> +	 * we can instead have drivers indicate whether they may have
>>>>>>> +	 * backing device map ranges and if not, skip this lookup.
>>>>>>> +	 */
>>>>>> I think we can do this statically:
>>>>>> - if it's system memory we know there's no devmap for it, and we do the
>>>>>>      trick to block gup_fast
>>>>> Yes, that should work.
>>>>>> - if it's iomem, we know gup_fast wont work anyway if don't set PFN_DEV,
>>>>>>      so might as well not do that
>>>>> I think gup_fast will unfortunately mistake a huge iomem page for an
>>>>> ordinary page and try to access a non-existant struct page for it, unless we
>>>>> do the devmap trick.
>>>>>
>>>>> And the lookup would then be for the rare case where a driver would have
>>>>> already registered a dev_pagemap for an iomem area which may also be mapped
>>>>> through TTM (like the patch from Felix a couple of weeks ago). If a driver
>>>>> can promise not to do that, then we can safely remove the lookup.
>>>> Isn't the devmap PTE flag arch optional? Does this fall back to not
>>>> using huge pages on arches that don't support it?
>>> Good point. No, currently it's only conditioned on transhuge page support.
>>> Need to condition it on also devmap support.
>>>
>>>> Also, I feel like this code to install "pte_special" huge pages does
>>>> not belong in the drm subsystem..
>>> I could add helpers in huge_memory.c:
>>>
>>> vmf_insert_pfn_pmd_prot_special() and
>>> vmf_insert_pfn_pud_prot_special()
>> The somewhat annoying thing is that we'd need an error code so we fall
>> back to pte fault handling. That's at least my understanding of how
>> pud/pmd fault handling works. Not sure how awkward that is going to be
>> with the overall fault handling flow.
>>
>> But aside from that I think this makes tons of sense.
> Why should the driver be so specific?
>
> vmf_insert_pfn_range_XXX()
>
> And it will figure out the optimal way to build the page tables.
>
> Driver should provide the largest physically contiguous range it can

I figure that would probably work, but since the huge_fault() interface 
is already providing the size of the fault based on how the pagetable is 
currently populated I figure that would have to move a lot of that logic 
into that helper...

/Thomas


>
> Jason