[PATCH v4 1/5] udmabuf: direct map pfn when first page fault

[Kasireddy, Vivek]

Hi Huan,

> Subject: [PATCH v4 1/5] udmabuf: direct map pfn when first page fault
> 
> The current udmabuf mmap uses a page fault 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.
> 
> As a result, the page fault 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,
> when creating a large size udmabuf, this represents a considerable
> overhead.
> 
> This patch fill vma area with pfn when the first page fault trigger, so,
> any other access will not enter page fault.
> 
> Suggested-by: Vivek Kasireddy <vivek.kasireddy at intel.com>
> Signed-off-by: Huan Yang <link at vivo.com>
> ---
>  drivers/dma-buf/udmabuf.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index 047c3cd2ceff..0e33d25310ec 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -43,7 +43,8 @@ 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;
> +	unsigned long addr, end, pfn;
> +	vm_fault_t ret;
> 
>  	if (pgoff >= ubuf->pagecount)
>  		return VM_FAULT_SIGBUS;
> @@ -51,7 +52,28 @@ static vm_fault_t udmabuf_vm_fault(struct vm_fault
> *vmf)
>  	pfn = folio_pfn(ubuf->folios[pgoff]);
>  	pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
> 
> -	return vmf_insert_pfn(vma, vmf->address, pfn);
> +	ret = vmf_insert_pfn(vma, vmf->address, pfn);
> +	if (ret & VM_FAULT_ERROR)
> +		return ret;
> +
> +	/* pre fault */
> +	pgoff = vma->vm_pgoff;
> +	end = vma->vm_end;
> +	addr = vma->vm_start;
> +
> +	for (; addr < end; pgoff++, addr += PAGE_SIZE) {
Although unlikely, I think we should also check for pgoff < ubuf->pagecount.

> +		if (addr == vmf->address)
> +			continue;
> +
> +		pfn = folio_pfn(ubuf->folios[pgoff]);
> +
> +		pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
> +
> +		if (vmf_insert_pfn(vma, addr, pfn) & VM_FAULT_ERROR)
Shouldn't you store the return value of vmf_insert_pfn in ret? Otherwise, we'll
return success when the above call fails.

Anyway, I am wondering if it is more optimal to just iterate over pages instead
of addresses. Something like below:

+       unsigned long nr_pages = vma_pages(vma);
+       unsigned long addr = vma->vm_start;

-       if (pgoff >= ubuf->pagecount)
-               return VM_FAULT_SIGBUS;
+       WARN_ON(nr_pages != ubuf->pagecount);

-       pfn = folio_pfn(ubuf->folios[pgoff]);
-       pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
+       for (pg = 0; pg < nr_pages && pg < ubuf->pagecount; pg++) {
+               pfn = folio_pfn(ubuf->folios[pg]);
+               pfn += ubuf->offsets[pg] >> PAGE_SHIFT;

-       return vmf_insert_pfn(vma, vmf->address, pfn);
+               ret = vmf_insert_pfn(vma, addr, pfn);
+               addr += PAGE_SIZE;
+ }

Thanks,
Vivek

> +			break;
> +	}
> +
> +	return ret;
>  }
> 
>  static const struct vm_operations_struct udmabuf_vm_ops = {
> --
> 2.45.2