RE: [PATCH v5 1/7] udmabuf: pre-fault when first page fault

[Kasireddy, Vivek]

Hi Huan,

> Subject: [PATCH v5 1/7] udmabuf: pre-fault 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 first the pfn into page table, and then pre-fault each pfn
> into vma, when first access. Should know, if anything wrong when
> pre-fault, will not report it's error, else, report when task access it
> at the first time.
> 
> Suggested-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
> Signed-off-by: Huan Yang <link@xxxxxxxx>
> ---
>  drivers/dma-buf/udmabuf.c | 35 +++++++++++++++++++++++++++++++++--
>  1 file changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index 047c3cd2ceff..0a8c231a36e1 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 +52,37 @@ 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;
Nit: use vma->vm_end directly in the loop below, as end is used only once.

> +	addr = vma->vm_start;
> +
> +	for (; addr < end; pgoff++, addr += PAGE_SIZE) {
> +		if (addr == vmf->address)
> +			continue;
> +
> +		if (WARN_ON(pgoff >= ubuf->pagecount))
> +			break;
> +
> +		pfn = folio_pfn(ubuf->folios[pgoff]);
> +
Nit: no need for a blank line here.

> +		pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
> +
> +		/**
> +		 * If something wrong, due to this vm fault success,
> +		 * do not report in here, report only when true access
> +		 * this addr.
> +		 * So, don't update ret here, just break.
Please rewrite the above comments as:
* If the below vmf_insert_pfn() fails, we do not return an error here
* during this pre-fault step. However, an error will be returned if the
* failure occurs when the addr is truly accessed.

With that,
Acked-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>

> +		 */
> +		if (vmf_insert_pfn(vma, addr, pfn) & VM_FAULT_ERROR)
> +			break;
> +	}
> +
> +	return ret;
>  }
> 
>  static const struct vm_operations_struct udmabuf_vm_ops = {
> --
> 2.45.2