[PATCH 10/13] dax: Prep dax_{associate, disassociate}_entry() for compound pages

[Dan Williams <dan.j.williams@xxxxxxxxx>]

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.

Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 drivers/dax/Kconfig   |    1 +
 drivers/dax/mapping.c |   18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index 3ed4da3935e5..2dcc8744277d 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -10,6 +10,7 @@ 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 0810af7d9503..6bd38ddba2cb 100644
--- a/drivers/dax/mapping.c
+++ b/drivers/dax/mapping.c
@@ -351,6 +351,8 @@ 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 {
@@ -358,6 +360,13 @@ static vm_fault_t dax_associate_entry(void *entry,
 			page->mapping = mapping;
 			page->index = index + i++;
 		}
+
+		/*
+		 * page->mapping and page->index are only manipulated on
+		 * head pages
+		 */
+		if (PageHead(page))
+			break;
 	}
 
 	return 0;
@@ -380,6 +389,8 @@ 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);
+
 		WARN_ON_ONCE(trunc && page_maybe_dma_pinned(page));
 		if (dax_mapping_is_cow(page->mapping)) {
 			/* keep the CoW flag if this page is still shared */
@@ -389,6 +400,13 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 			WARN_ON_ONCE(page->mapping && page->mapping != mapping);
 		page->mapping = NULL;
 		page->index = 0;
+
+		/*
+		 * page->mapping and page->index are only manipulated on
+		 * head pages
+		 */
+		if (PageHead(page))
+			break;
 	}
 }