On Mon, Mar 01, 2021 at 09:39:53AM +0100, Thomas Hellström (Intel) wrote: > Hi, > > On 3/1/21 9:28 AM, Daniel Vetter wrote: > > 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 > > Hmm perhaps it might, but I don't think so. The fix I tried out was to set > > PFN_DEV | PFN_MAP for huge PTEs which causes pfn_devmap() to be true, and > then > > follow_devmap_pmd()->get_dev_pagemap() which returns NULL and gup_fast() > backs off, > > in the end that would mean setting in stone that "if there is a huge devmap > page table entry for which we haven't registered any devmap struct pages > (get_dev_pagemap returns NULL), we should treat that as a "special" huge > page table entry". > > From what I can tell, all code calling get_dev_pagemap() already does that, > it's just a question of getting it accepted and formalizing it. Oh I thought that's already how it works, since I didn't spot anything else that would block gup_fast from falling over. I guess really would need some testcases to make sure direct i/o (that's the easiest to test) fails like we expect. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
