Messages in this thread | | | Subject | Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages | From | Thomas Hellström (Intel) <> | Date | Wed, 24 Mar 2021 13:35:17 +0100 |
| |
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
| |