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 1:41 PM, Jason Gunthorpe wrote:
> On Wed, Mar 24, 2021 at 01:35:17PM +0100, Thomas Hellström (Intel) wrote:
>> 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...
> But we don't really care about the size of the fault when we stuff the
> pfns.
>
> The device might use it when handling the fault, but once the fault is
> handled the device knows what the contiguous pfn range is that it has
> available to stuff into the page tables, it just tells the vmf_insert
> what it was able to create, and it creates the necessary page table
> structure.
>
> The size of the hole in the page table is really only advisory, the
> device may not want to make a 2M or 1G page entry and may prefer to
> only create 4k.
>
> In an ideal world the creation/destruction of page table levels would
> by dynamic at this point, like THP.

Hmm, but I'm not sure what problem we're trying to solve by changing the 
interface in this way?

Currently if the core vm requests a huge pud, we give it one, and if we 
can't or don't want to (because of dirty-tracking, for example, which is 
always done on 4K page-level) we just return VM_FAULT_FALLBACK, and the 
fault is retried at a lower level. Also, determining whether we have a 
contigous range is not free, so we  don't want to do that unnecessarily.

/Thomas