Re: [Linaro-mm-sig] [PATCH 1/2] dma-buf: Require VM_PFNMAP vma for mmap

Daniel Vetter
2021-03-01T09:07:05Z

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@shipmail.org> 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