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

[Huan Yang]

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 at intel.com>
Signed-off-by: Huan Yang <link at vivo.com>
---
 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;
+	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]);
+
+		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.
+		 */
+		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