[PATCH v6 10/10] device-dax: compound devmap support

[Joao Martins <joao.m.martins@oracle.com>]

Use the newly added compound devmap facility which maps the assigned dax
ranges as compound pages at a page size of @align.

dax devices are created with a fixed @align (huge page size) which is
enforced through as well at mmap() of the device. Faults, consequently
happen too at the specified @align specified at the creation, and those
don't change throughout dax device lifetime. MCEs unmap a whole dax
huge page, as well as splits occurring at the configured page size.

Performance measured by gup_test improves considerably for
unpin_user_pages() and altmap with NVDIMMs:

$ gup_test -f /dev/dax1.0 -m 16384 -r 10 -S -a -n 512 -w
(pin_user_pages_fast 2M pages) put:~71 ms -> put:~22 ms
[altmap]
(pin_user_pages_fast 2M pages) get:~524ms put:~525 ms -> get: ~127ms put:~71ms

 $ gup_test -f /dev/dax1.0 -m 129022 -r 10 -S -a -n 512 -w
(pin_user_pages_fast 2M pages) put:~513 ms -> put:~188 ms
[altmap with -m 127004]
(pin_user_pages_fast 2M pages) get:~4.1 secs put:~4.12 secs -> get:~1sec put:~563ms

.. as well as unpin_user_page_range_dirty_lock() being just as effective
as THP/hugetlb[0] pages.

[0] https://lore.kernel.org/linux-mm/20210212130843.13865-5-joao.m.martins@oracle.com/

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/dax/device.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index 0ef9fecec005..9b51108aea91 100644
--- a/drivers/dax/device.c
+++ b/drivers/dax/device.c
@@ -78,14 +78,20 @@ static void dax_set_mapping(struct vm_fault *vmf, pfn_t pfn,
 {
 	unsigned long i, nr_pages = fault_size / PAGE_SIZE;
 	struct file *filp = vmf->vma->vm_file;
+	struct dev_dax *dev_dax = filp->private_data;
 	pgoff_t pgoff;
 
+	/* mapping is only set on the head */
+	if (dev_dax->pgmap->vmemmap_shift)
+		nr_pages = 1;
+
 	pgoff = linear_page_index(vmf->vma,
 			ALIGN(vmf->address, fault_size));
 
 	for (i = 0; i < nr_pages; i++) {
 		struct page *page = pfn_to_page(pfn_t_to_pfn(pfn) + i);
 
+		page = compound_head(page);
 		if (page->mapping)
 			continue;
 
@@ -445,6 +451,9 @@ int dev_dax_probe(struct dev_dax *dev_dax)
 	}
 
 	pgmap->type = MEMORY_DEVICE_GENERIC;
+	if (dev_dax->align > PAGE_SIZE)
+		pgmap->vmemmap_shift =
+			order_base_2(dev_dax->align >> PAGE_SHIFT);
 	addr = devm_memremap_pages(dev, pgmap);
 	if (IS_ERR(addr))
 		return PTR_ERR(addr);
-- 
2.17.2