Re: [PATCH v2 1/4] udmabuf: cancel mmap page fault, direct map it

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

Am 12.08.24 um 04:49 schrieb Huan Yang:
>
> 在 2024/8/10 9:28, Kasireddy, Vivek 写道:
>> [Some people who received this message don't often get email from 
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>>
>> Hi Huan,
>>
>>> The current udmabuf mmap uses a page fault mechanism to populate the
>>> vma.
>>>
>>> However, the current udmabuf has already obtained and pinned the folio
>>> upon completion of the creation.This means that the physical memory has
>>> already been acquired, rather than being accessed dynamically. The
>>> current page fault method only saves some page table memory.
>>>
>>> As a result, the page fault mechanism has lost its purpose as a 
>>> demanding
>>> page. Due to the fact that page fault requires trapping into kernel 
>>> mode
>>> and filling in when accessing the corresponding virtual address in 
>>> mmap,
>>> this means that user mode access to virtual addresses needs to trap 
>>> into
>>> kernel mode.
>>>
>>> Therefore, when creating a large size udmabuf, this represents a
>>> considerable overhead.
>>>
>>> The current patch removes the page fault method of mmap and
>>> instead fills it directly when mmap is triggered.
>> I think it makes sense to populate the vma when the first fault is 
>> triggered
>> instead of doing it during mmap. This is because the userspace may call
>> mmap but does not actually use the data. Qemu works this way 
>> depending on
> Yes, the idea of this is also related to the concept of page fault.
>
> However, the folio has already been pinned during creation. I think 
> using the page fault
>
> again is theoretically sound, but it may not save memory, only 
> increase context switch overhead.

This is not about saving memory but rather correctness and desired handling.

A mmap() operation is for creating the VMA and *not* filling the page 
tables. That might work but is not really a desired approach.

Regards,
Christian.

>
>
>> whether opengl is available in the environment or not.
>>
>>> Signed-off-by: Huan Yang <link@vivo.com>
>>> ---
>>>   drivers/dma-buf/udmabuf.c | 39 
>>> ++++++++++++++++-----------------------
>>>   1 file changed, 16 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
>>> index 047c3cd2ceff..475268d4ebb1 100644
>>> --- a/drivers/dma-buf/udmabuf.c
>>> +++ b/drivers/dma-buf/udmabuf.c
>>> @@ -38,36 +38,29 @@ struct udmabuf_folio {
>>>        struct list_head list;
>>>   };
>>>
>>> -static vm_fault_t udmabuf_vm_fault(struct vm_fault *vmf)
>>> -{
>>> -     struct vm_area_struct *vma = vmf->vma;
>>> -     struct udmabuf *ubuf = vma->vm_private_data;
>>> -     pgoff_t pgoff = vmf->pgoff;
>>> -     unsigned long pfn;
>>> -
>>> -     if (pgoff >= ubuf->pagecount)
>>> -             return VM_FAULT_SIGBUS;
>>> -
>>> -     pfn = folio_pfn(ubuf->folios[pgoff]);
>>> -     pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
>>> -
>>> -     return vmf_insert_pfn(vma, vmf->address, pfn);
>>> -}
>>> -
>>> -static const struct vm_operations_struct udmabuf_vm_ops = {
>>> -     .fault = udmabuf_vm_fault,
>>> -};
>>> -
>>>   static int mmap_udmabuf(struct dma_buf *buf, struct vm_area_struct
>>> *vma)
>>>   {
>>>        struct udmabuf *ubuf = buf->priv;
>>> +     unsigned long addr;
>>> +     unsigned long end;
>>> +     unsigned long pgoff;
>>> +     int ret;
>>>
>>>        if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
>>>                return -EINVAL;
>>>
>>> -     vma->vm_ops = &udmabuf_vm_ops;
>>> -     vma->vm_private_data = ubuf;
>>> -     vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND |
>>> VM_DONTDUMP);
>>> +     for (pgoff = vma->vm_pgoff, end = vma->vm_end, addr = vma-
>>>> vm_start;
>>> +          addr < end; pgoff++, addr += PAGE_SIZE) {
>>> +             struct page *page =
>>> +                     folio_page(ubuf->folios[pgoff],
>>> +                                ubuf->offsets[pgoff] >> PAGE_SHIFT);
>> Please don't use struct page pointers, given the recent conversion to 
>> use
>> only folios in udmabuf driver. I think what you are trying to do 
>> above can
>> be done using only folios.
> Yes, just use pfn. Consider of HVO, must use this.
>>
>>> +
>>> +             ret = remap_pfn_range(vma, addr, page_to_pfn(page),
>>> PAGE_SIZE,
>>> +                                   vma->vm_page_prot);
>> Could you please retain the use of vmf_insert_pfn() here, given the 
>> simplicity,
>> among other reasons?
> I will make the correction.
>
> Thanks.
>>
>> Thanks,
>> Vivek
>>
>>> +             if (ret)
>>> +                     return ret;
>>> +     }
>>> +
>>>        return 0;
>>>   }
>>>
>>> -- 
>>> 2.45.2