[PATCH v6 1/7] udmabuf: pre-fault when first page fault

[Huan Yang]

The current udmabuf mmap only fills the physical memory to the
corresponding virtual address when the user actually accesses the
virtual address.

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 the pfn into page table, and then pre-fault each pfn
into vma, when first access.

Notice, if anything wrong , we do not return an error during this
pre-fault step. However, an error will be returned if the failure occurs
when the addr is truly accessed

Suggested-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
Signed-off-by: Huan Yang <link@xxxxxxxx>
Acked-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
---
 drivers/dma-buf/udmabuf.c | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index 047c3cd2ceff..2170d975cc76 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, pfn;
+	vm_fault_t ret;
 
 	if (pgoff >= ubuf->pagecount)
 		return VM_FAULT_SIGBUS;
@@ -51,7 +52,35 @@ 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;
+	addr = vma->vm_start;
+
+	for (; addr < vma->vm_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 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.
+		 */
+		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