Re: [RFC PATCH 2/2] mm,drm/ttm: Use VM_PFNMAP for TTM vmas

["Christian König" <christian.koenig@amd.com>]

Am 22.03.21 um 09:13 schrieb Thomas Hellström (Intel):
> Hi!
>
> On 3/22/21 8:47 AM, Christian König wrote:
>> Am 21.03.21 um 19:45 schrieb Thomas Hellström (Intel):
>>> To block fast gup we need to make sure TTM ptes are always special.
>>> With MIXEDMAP we, on architectures that don't support pte_special,
>>> insert normal ptes, but OTOH on those architectures, fast is not
>>> supported.
>>> At the same time, the function documentation to vm_normal_page() 
>>> suggests
>>> that ptes pointing to system memory pages of MIXEDMAP vmas are always
>>> normal, but that doesn't seem consistent with what's implemented in
>>> vmf_insert_mixed(). I'm thus not entirely sure this patch is actually
>>> needed.
>>>
>>> But to make sure and to avoid also normal (non-fast) gup, make all
>>> TTM vmas PFNMAP. With PFNMAP we can't allow COW mappings
>>> anymore so make is_cow_mapping() available and use it to reject
>>> COW mappigs at mmap time.
>>
>> I would separate the disallowing of COW mapping from the PFN change. 
>> I'm pretty sure that COW mappings never worked on TTM BOs in the 
>> first place.
>
> COW doesn't work with PFNMAP together with non-linear maps, so as a 
> consequence from moving from MIXEDMAP to PFNMAP we must disallow COW, 
> so it seems logical to me to do it in one patch.
>
> And working COW was one of the tests I used for huge PMDs/PUDs, so it 
> has indeed been working, but I can't think of any relevant use-cases.

Ok, going to keep that in mind. I was assuming COW didn't worked before 
on TTM pages.

> Did you, BTW, have a chance to test this with WC mappings?

I'm going to give this a full piglit round, but currently I'm busy with 
internal testing.

Thanks,
Christian.

>
> Thanks,
> /Thomas
>
>
>
>>
>> But either way this patch is Reviewed-by: Christian König 
>> <christian.koenig@amd.com>.
>>
>> Thanks,
>> Christian.
>>
>>>
>>> There was previously a comment in the code that WC mappings together
>>> with x86 PAT + PFNMAP was bad for performance. However from looking at
>>> vmf_insert_mixed() it looks like in the current code PFNMAP and 
>>> MIXEDMAP
>>> are handled the same for architectures that support pte_special. This
>>> means there should not be a performance difference anymore, but this
>>> needs to be verified.
>>>
>>> Cc: Christian Koenig <christian.koenig@amd.com>
>>> Cc: David Airlie <airlied@linux.ie>
>>> Cc: Daniel Vetter <daniel@ffwll.ch>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Jason Gunthorpe <jgg@nvidia.com>
>>> Cc: linux-mm@kvack.org
>>> Cc: dri-devel@lists.freedesktop.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Signed-off-by: Thomas Hellström (Intel) <thomas_os@shipmail.org>
>>> ---
>>>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 22 ++++++++--------------
>>>   include/linux/mm.h              |  5 +++++
>>>   mm/internal.h                   |  5 -----
>>>   3 files changed, 13 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c 
>>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>> index 1c34983480e5..708c6fb9be81 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>> @@ -372,12 +372,7 @@ 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))) {
>>> @@ -555,18 +550,14 @@ static void ttm_bo_mmap_vma_setup(struct 
>>> ttm_buffer_object *bo, struct vm_area_s
>>>        * Note: We're transferring the bo reference to
>>>        * vma->vm_private_data here.
>>>        */
>>> -
>>>       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
>>> +     * PFNMAP forces us to block COW mappings in mmap(),
>>> +     * and with MIXEDMAP we would incorrectly allow fast gup
>>> +     * on TTM memory on architectures that don't have pte_special.
>>>        */
>>> -    vma->vm_flags |= VM_MIXEDMAP;
>>> -    vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
>>> +    vma->vm_flags |= VM_PFNMAP | VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
>>>   }
>>>     int ttm_bo_mmap(struct file *filp, struct vm_area_struct *vma,
>>> @@ -579,6 +570,9 @@ int ttm_bo_mmap(struct file *filp, struct 
>>> vm_area_struct *vma,
>>>       if (unlikely(vma->vm_pgoff < DRM_FILE_PAGE_OFFSET_START))
>>>           return -EINVAL;
>>>   +    if (unlikely(is_cow_mapping(vma->vm_flags)))
>>> +        return -EINVAL;
>>> +
>>>       bo = ttm_bo_vm_lookup(bdev, vma->vm_pgoff, vma_pages(vma));
>>>       if (unlikely(!bo))
>>>           return -EINVAL;
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index 77e64e3eac80..c6ebf7f9ddbb 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -686,6 +686,11 @@ static inline bool vma_is_accessible(struct 
>>> vm_area_struct *vma)
>>>       return vma->vm_flags & VM_ACCESS_FLAGS;
>>>   }
>>>   +static inline bool is_cow_mapping(vm_flags_t flags)
>>> +{
>>> +    return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
>>> +}
>>> +
>>>   #ifdef CONFIG_SHMEM
>>>   /*
>>>    * The vma_is_shmem is not inline because it is used only by slow
>>> diff --git a/mm/internal.h b/mm/internal.h
>>> index 9902648f2206..1432feec62df 100644
>>> --- a/mm/internal.h
>>> +++ b/mm/internal.h
>>> @@ -296,11 +296,6 @@ static inline unsigned int buddy_order(struct 
>>> page *page)
>>>    */
>>>   #define buddy_order_unsafe(page) READ_ONCE(page_private(page))
>>>   -static inline bool is_cow_mapping(vm_flags_t flags)
>>> -{
>>> -    return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
>>> -}
>>> -
>>>   /*
>>>    * These three helpers classifies VMAs for virtual memory accounting.
>>>    */