[PATCH v2 13/18] dax: Prep mapping helpers for compound pages

[Dan Williams <dan.j.williams@intel.com>]

In preparation for device-dax to use the same mapping machinery as
fsdax, add support for device-dax compound pages.

Presently this is handled by dax_set_mapping() which is careful to only
update page->mapping for head pages. However, it does that by looking at
properties in the 'struct dev_dax' instance associated with the page.
Switch to just checking PageHead() directly in the functions that
iterate over pages in a large mapping.

Cc: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/dax/Kconfig   |    1 +
 drivers/dax/mapping.c |   16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index 205e9dda8928..2eddd32c51f4 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -9,6 +9,7 @@ if DAX
 config DEV_DAX
 	tristate "Device DAX: direct access mapping device"
 	depends on TRANSPARENT_HUGEPAGE
+	depends on !FS_DAX_LIMITED
 	help
 	  Support raw access to differentiated (persistence, bandwidth,
 	  latency...) memory via an mmap(2) capable character
diff --git a/drivers/dax/mapping.c b/drivers/dax/mapping.c
index 70576aa02148..5d4b9601f183 100644
--- a/drivers/dax/mapping.c
+++ b/drivers/dax/mapping.c
@@ -345,6 +345,8 @@ static vm_fault_t dax_associate_entry(void *entry,
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
+		page = compound_head(page);
+
 		if (flags & DAX_COW) {
 			dax_mapping_set_cow(page);
 		} else {
@@ -353,6 +355,9 @@ static vm_fault_t dax_associate_entry(void *entry,
 			page->index = index + i++;
 			page_ref_inc(page);
 		}
+
+		if (PageHead(page))
+			break;
 	}
 
 	return 0;
@@ -372,6 +377,9 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 
 	for_each_mapped_pfn(entry, pfn) {
 		page = pfn_to_page(pfn);
+
+		page = compound_head(page);
+
 		if (dax_mapping_is_cow(page->mapping)) {
 			/* keep the CoW flag if this page is still shared */
 			if (page->index-- > 0)
@@ -383,6 +391,9 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 		}
 		page->mapping = NULL;
 		page->index = 0;
+
+		if (PageHead(page))
+			break;
 	}
 
 	if (trunc && !dax_mapping_is_cow(page->mapping)) {
@@ -660,11 +671,16 @@ static struct page *dax_zap_pages(struct xa_state *xas, void *entry)
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
+		page = compound_head(page);
+
 		if (zap)
 			page_ref_dec(page);
 
 		if (!ret && !dax_page_idle(page))
 			ret = page;
+
+		if (PageHead(page))
+			break;
 	}
 
 	if (zap)