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

Daniel Vetter
2021-03-01T08:30:01Z

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
--
Daniel Vetter
Software Engineer, Intel Corporation