[PATCH] drm/ttm: nuke VM_MIXEDMAP on BO mappings v2

[Christian König]

Am 07.06.21 um 18:13 schrieb Thomas Hellström (Intel):
>
> On 6/7/21 3:58 PM, Christian König wrote:
>> We discussed if that is really the right approach for quite a while 
>> now, but
>> digging deeper into a bug report on arm turned out that this is actually
>> horrible broken right now.
>>
>> The reason for this is that vmf_insert_mixed_prot() always tries to grab
>> a reference to the underlaying page on architectures without
>> ARCH_HAS_PTE_SPECIAL and as far as I can see also enabled GUP.
>>
>> So nuke using VM_MIXEDMAP here and use VM_PFNMAP instead.
>>
>> Also make sure to reject mappings without VM_SHARED.
>>
>> v2: reject COW mappings, merge function with only caller
>>
>> Signed-off-by: Christian König <christian.koenig at amd.com>
>> Bugs: https://gitlab.freedesktop.org/drm/amd/-/issues/1606#note_936174
>> ---
>>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 44 +++++++++++----------------------
>>   1 file changed, 14 insertions(+), 30 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c 
>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> index 61828488ae2b..c9edb75626d9 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> @@ -359,12 +359,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct 
>> vm_fault *vmf,
>>            * at arbitrary times while the data is mmap'ed.
>>            * See vmf_insert_mixed_prot() for a discussion.
>>            */
>> -        if (vma->vm_flags & VM_MIXEDMAP)
>> -            ret = vmf_insert_mixed_prot(vma, address,
>> -                            __pfn_to_pfn_t(pfn, PFN_DEV),
>> -                            prot);
>> -        else
>> -            ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>> +        ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>>             /* Never error on prefaulted PTEs */
>>           if (unlikely((ret & VM_FAULT_ERROR))) {
>> @@ -411,15 +406,9 @@ vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault 
>> *vmf, pgprot_t prot)
>>       pfn = page_to_pfn(page);
>>         /* Prefault the entire VMA range right away to avoid further 
>> faults */
>> -    for (address = vma->vm_start; address < vma->vm_end; address += 
>> PAGE_SIZE) {
>> -
>> -        if (vma->vm_flags & VM_MIXEDMAP)
>> -            ret = vmf_insert_mixed_prot(vma, address,
>> -                            __pfn_to_pfn_t(pfn, PFN_DEV),
>> -                            prot);
>> -        else
>> -            ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>> -    }
>> +    for (address = vma->vm_start; address < vma->vm_end;
>> +         address += PAGE_SIZE)
>> +        ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>>         return ret;
>>   }
>> @@ -560,8 +549,16 @@ static const struct vm_operations_struct 
>> ttm_bo_vm_ops = {
>>       .access = ttm_bo_vm_access,
>>   };
>>   -static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, 
>> struct vm_area_struct *vma)
>> +int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct 
>> ttm_buffer_object *bo)
>>   {
>> +    /* Enforce VM_SHARED here since without it we would have really 
>> strange
>> +     * behavior on COW.
>> +     */
>
> Nit: Perhaps "Enforce no COW.." since mappings are allowed with 
> VM_SHARED iff VM_MAYWRITE is not set. Also style consistency with 
> comments: First /* followed by line-break or are you adapting the 
> above style for ttm?

Good point and no not really. I just sometimes forget to hit enter here 
and we don't have any automated script complaining.

>
> With that fixed,
>
> Reviewed-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>

Thanks,
Christian.

>
>
>> +    if (is_cow_mapping(vma->vm_flags))
>> +        return -EINVAL;
>> +
>> +    ttm_bo_get(bo);
>> +
>>       /*
>>        * Drivers may want to override the vm_ops field. Otherwise we
>>        * use TTM's default callbacks.
>> @@ -576,21 +573,8 @@ 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
>> -     */
>> -    vma->vm_flags |= VM_MIXEDMAP;
>> +    vma->vm_flags |= VM_PFNMAP;
>>       vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
>> -}
>> -
>> -int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct 
>> ttm_buffer_object *bo)
>> -{
>> -    ttm_bo_get(bo);
>> -    ttm_bo_mmap_vma_setup(bo, vma);
>>       return 0;
>>   }
>>   EXPORT_SYMBOL(ttm_bo_mmap_obj);