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