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. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
