[Linaro-mm-sig] Re: [PATCH 1/5] udmabuf: cancel mmap page fault, direct map it

Am 01.08.24 um 12:45 schrieb Huan Yang:

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.

Therefore, the current patch removes the page fault method of mmap and instead fills it directly when mmap is triggered.

Signed-off-by: Huan Yang link@vivo.com

drivers/dma-buf/udmabuf.c | 70 ++++++++++++++++++++++----------------- 1 file changed, 39 insertions(+), 31 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c index 047c3cd2ceff..d69aeada7367 100644 --- a/drivers/dma-buf/udmabuf.c +++ b/drivers/dma-buf/udmabuf.c @@ -38,36 +38,39 @@ 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 struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,

• 		     enum dma_data_direction direction);
  

static int mmap_udmabuf(struct dma_buf *buf, struct vm_area_struct *vma) { struct udmabuf *ubuf = buf->priv;

• struct sg_table *table = ubuf->sg;
• unsigned long addr = vma->vm_start;
• struct sg_page_iter piter;
• 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);

• if (!table) {

• table = get_sg_table(NULL, buf, 0);
  

• if (IS_ERR(table))
  

• 	return PTR_ERR(table);
  

• ubuf->sg = table;
  

• }
• for_each_sgtable_page(table, &piter, vma->vm_pgoff) {

That might not work correctly. We intentionally remove the pages from the sgtable when it is shared between devices.

Additional to that the sgtable is *not* a page container, but rather a DMA address container. So that here is also a rather bad idea from the design side.

Regards, Christian.

• struct page *page = sg_page_iter_page(&piter);
  

• ret = remap_pfn_range(vma, addr, page_to_pfn(page), PAGE_SIZE,
  

• 		      vma->vm_page_prot);
  

• if (ret)
  

• 	return ret;
  

• addr += PAGE_SIZE;
  

• if (addr >= vma->vm_end)
  

• 	return 0;
  

• }
• return 0; }

@@ -126,6 +129,10 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf, sg_set_folio(sgl, ubuf->folios[i], PAGE_SIZE, ubuf->offsets[i]);

• // if dev is NULL, no need to sync.
• if (!dev)

• return sg;
  

• ret = dma_map_sgtable(dev, sg, direction, 0); if (ret < 0) goto err_map;

@@ -206,20 +213,21 @@ static int begin_cpu_udmabuf(struct dma_buf *buf, { struct udmabuf *ubuf = buf->priv; struct device *dev = ubuf->device->this_device;

• int ret = 0;

• struct sg_table *sg;

• if (!ubuf->sg) {

• ubuf->sg = get_sg_table(dev, buf, direction);
  

• if (IS_ERR(ubuf->sg)) {
  

• 	ret = PTR_ERR(ubuf->sg);
  

• 	ubuf->sg = NULL;
  

• }
  

• } else {

• if (ubuf->sg) { dma_sync_sg_for_cpu(dev, ubuf->sg->sgl, ubuf->sg->nents, direction);

• return 0;
  

  }

• return ret;

• sg = get_sg_table(dev, buf, direction);
• if (IS_ERR(sg))

• return PTR_ERR(sg);
  

• ubuf->sg = sg;
• return 0; }

static int end_cpu_udmabuf(struct dma_buf *buf,