On Sat, Feb 27, 2021 at 9:06 AM Thomas Hellström (Intel) <thomas_os@xxxxxxxxxxxx> wrote: > On 2/26/21 2:28 PM, Daniel Vetter wrote: > > So I think it stops gup. But I haven't verified at all. Would be good > > if Christian can check this with some direct io to a buffer in system > > memory. > > Hmm, > > Docs (again vm_normal_page() say) > > * VM_MIXEDMAP mappings can likewise contain memory with or without "struct > * page" backing, however the difference is that _all_ pages with a struct > * page (that is, those where pfn_valid is true) are refcounted and > considered > * normal pages by the VM. The disadvantage is that pages are refcounted > * (which can be slower and simply not an option for some PFNMAP > users). The > * advantage is that we don't have to follow the strict linearity rule of > * PFNMAP mappings in order to support COWable mappings. > > but it's true __vm_insert_mixed() ends up in the insert_pfn() path, so > the above isn't really true, which makes me wonder if and in that case > why there could any longer ever be a significant performance difference > between MIXEDMAP and PFNMAP. Yeah it's definitely confusing. I guess I'll hack up a patch and see what sticks. > BTW regarding the TTM hugeptes, I don't think we ever landed that devmap > hack, so they are (for the non-gup case) relying on > vma_is_special_huge(). For the gup case, I think the bug is still there. Maybe there's another devmap hack, but the ttm_vm_insert functions do use PFN_DEV and all that. And I think that stops gup_fast from trying to find the underlying page. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
