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@xxxxxxxxx>
> Signed-off-by: Huan Yang <link@xxxxxxxx>
> ---
> 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
