[PATCH] drm/ttm: nuke VM_MIXEDMAP on BO mappings

[Christian König]

Am 02.06.21 um 13:24 schrieb Thomas Hellström (Intel):
> [SNIP]
>>>> @@ -576,14 +565,10 @@ static void ttm_bo_mmap_vma_setup(struct 
>>>> ttm_buffer_object *bo, struct vm_area_s
>>>>         vma->vm_private_data = bo;
>>>>   -    /*
>>>> -     * We'd like to use VM_PFNMAP on shared mappings, where
>>>> -     * (vma->vm_flags & VM_SHARED) != 0, for performance reasons,
>>>> -     * but for some reason VM_PFNMAP + x86 PAT + write-combine is 
>>>> very
>>>> -     * bad for performance. Until that has been sorted out, use
>>>> -     * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
>>>> +    /* Enforce VM_SHARED here since no driver backend actually 
>>>> supports COW
>>>> +     * on TTM buffer object mappings.
>>>
>>> I think by default all TTM drivers support COW mappings in the sense 
>>> that written data never makes it to the bo but stays in anonymous 
>>> pages, although I can't find a single usecase. So comment should be 
>>> changed to state that they are useless for us and that we can't 
>>> support COW mappings with VM_PFNMAP.
>>
>> Well the problem I see with that is that it only works as long as the 
>> BO is in system memory. When it then suddenly migrates to VRAM 
>> everybody sees the same content again and the COW pages are dropped. 
>> That is really inconsistent and I can't see why we would want to do 
>> that.
> Hmm, yes, that's actually a bug in drm_vma_manager().

Hui? How is that related to drm_vma_manager() ?

>>
>> Additionally to that when you allow COW mappings you need to make 
>> sure your COWed pages have the right caching attribute and that the 
>> reference count is initialized and taken into account properly. Not 
>> driver actually gets that right at the moment.
>
> I was under the impression that COW'ed pages were handled 
> transparently by the vm, you'd always get cached properly refcounted 
> COW'ed pages but anyway since we're going to ditch support for them, 
> doesn't really matter.

Yeah, but I would have expected that the new COWed page should have the 
same caching attributes as the old one and that is not really the case.

>
>>
>>>
>>>>        */
>>>> -    vma->vm_flags |= VM_MIXEDMAP;
>>>> +    vma->vm_flags |= VM_PFNMAP | VM_SHARED;
>>>
>>> Hmm, shouldn't we refuse COW mappings instead, like my old patch on 
>>> this subject did? In theory someone could be setting up what she 
>>> thinks is a private mapping to a shared buffer object, and write 
>>> sensitive data to it, which will immediately leak. It's a simple 
>>> check, could open-code if necessary.
>>
>> Yeah, though about that as well. Rejecting things would mean we 
>> potentially break userspace which just happened to work by coincident 
>> previously. Not totally evil, but not nice either.
>>
>> How about we do a WARN_ON_ONCE(!(vma->vm_flags & VM_SHARED)); instead?
>
> Umm, yes but that wouldn't notify the user, and would be triggerable 
> from user-space. But you can also set up legal non-COW mappings 
> without the VM_SHARED flag, IIRC, see is_cow_mapping(). I think when 
> this was up for discussion last time we arrived in a 
> vma_is_cow_mapping() utility...

Well userspace could trigger that only once, so no spamming of the log 
can be expected here. And extra warnings in the logs are usually 
reported by people rather quickly.

Christian.

>
> /Thomas
>
>