udmabuf: pre-fault when first page fault

JIRA: https://issues.redhat.com/browse/RHEL-89519

commit f0bbcc2
Author: Huan Yang <link@vivo.com>
Date:   Wed, 18 Sep 2024 10:52:24 +0800

    udmabuf: pre-fault when first page fault

    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@intel.com>
    Signed-off-by: Huan Yang <link@vivo.com>
    Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
    Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20240918025238.2957823-2-link@vivo.com

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

drivers/dma-buf/udmabuf.c

@@ -43,15 +43,44 @@ 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;
 
 	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 = {