On 11/12/21 16:34, Jason Gunthorpe wrote:

On Fri, Nov 12, 2021 at 04:08:24PM +0100, Joao Martins wrote:

quoted

diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index a65c67ab5ee0..0c2ac97d397d 100644
+++ b/drivers/dax/device.c

@@ -192,6 +192,42 @@ static vm_fault_t __dev_dax_pud_fault(struct dev_dax *dev_dax,
 }
 #endif /* !CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
 
+static void set_page_mapping(struct vm_fault *vmf, pfn_t pfn,
+			     unsigned long fault_size,
+			     struct address_space *f_mapping)
+{
+	unsigned long i;
+	pgoff_t pgoff;
+
+	pgoff = linear_page_index(vmf->vma, ALIGN(vmf->address, fault_size));
+
+	for (i = 0; i < fault_size / PAGE_SIZE; i++) {
+		struct page *page;
+
+		page = pfn_to_page(pfn_t_to_pfn(pfn) + i);
+		if (page->mapping)
+			continue;
+		page->mapping = f_mapping;
+		page->index = pgoff + i;
+	}
+}
+
+static void set_compound_mapping(struct vm_fault *vmf, pfn_t pfn,
+				 unsigned long fault_size,
+				 struct address_space *f_mapping)
+{
+	struct page *head;
+
+	head = pfn_to_page(pfn_t_to_pfn(pfn));
+	head = compound_head(head);
+	if (head->mapping)
+		return;
+
+	head->mapping = f_mapping;
+	head->index = linear_page_index(vmf->vma,
+			ALIGN(vmf->address, fault_size));
+}

Should this stuff be setup before doing vmf_insert_pfn_XX?

Interestingly filesystem-dax does this, but not device-dax.

set_page_mapping/set_compound_mapping() could be moved to before and then torn down
on @rc != VM_FAULT_NOPAGE (failure). I am not sure what's the benefit in this series..
besides the ordering (that you hinted below) ?

In normal cases the page should be returned in the vmf and populated
to the page tables by the core code after all this is done. 

So I suppose by call sites examples as 'core code' is either hugetlbfs call to
__filemap_add_folio() (on hugetlbfs fault handler), shmem_add_to_page_cache() or
anon-equivalent.