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

Thomas Hellström (Intel)
2021-03-01T09:22:09Z

On 3/1/21 10:05 AM, Daniel Vetter wrote:
> 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.

Yeah, IIRC the "| PFN_MAP" is the missing piece for TTM huge ptes.
Otherwise pmd_devmap() will not return true and since there is no
pmd_special() things break.

/Thomas

> -Daniel