[RFC PATCH v2 04/12] dev_dax_iomap: Save the kva from memremap

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Save the kva from memremap because we need it for iomap rw support.

Prior to famfs, there were no iomap users of /dev/dax - so the virtual
address from memremap was not needed.

Also: in some cases dev_dax_probe() is called with the first
dev_dax->range offset past the start of pgmap[0].range. In those cases
we need to add the difference to virt_addr in order to have the physaddr's
in dev_dax->ranges match dev_dax->virt_addr.

This happens with devdax devices that started as pmem and got converted
to devdax. I'm not sure whether the offset is due to label storage, or
page tables, but this works in all known cases.

Signed-off-by: John Groves <john@xxxxxxxxxx>
 drivers/dax/dax-private.h |  1 +
 drivers/dax/device.c      | 15 +++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
index 446617b73aea..df5b3d975df4 100644
--- a/drivers/dax/dax-private.h
+++ b/drivers/dax/dax-private.h
@@ -63,6 +63,7 @@ struct dax_mapping {
 struct dev_dax {
 	struct dax_region *region;
 	struct dax_device *dax_dev;
+	void *virt_addr;
 	unsigned int align;
 	int target_node;
 	bool dyn_id;
diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index 40ba660013cf..17323b5f6f57 100644
--- a/drivers/dax/device.c
+++ b/drivers/dax/device.c
@@ -372,6 +372,7 @@ static int dev_dax_probe(struct dev_dax *dev_dax)
 	struct dax_device *dax_dev = dev_dax->dax_dev;
 	struct device *dev = &dev_dax->dev;
 	struct dev_pagemap *pgmap;
+	u64 data_offset = 0;
 	struct inode *inode;
 	struct cdev *cdev;
 	void *addr;
@@ -426,6 +427,20 @@ static int dev_dax_probe(struct dev_dax *dev_dax)
 	if (IS_ERR(addr))
 		return PTR_ERR(addr);
+	/* Detect whether the data is at a non-zero offset into the memory */
+	if (pgmap->range.start != dev_dax->ranges[0].range.start) {
+		u64 phys = dev_dax->ranges[0].range.start;
+		u64 pgmap_phys = dev_dax->pgmap[0].range.start;
+		u64 vmemmap_shift = dev_dax->pgmap[0].vmemmap_shift;
+		if (!WARN_ON(pgmap_phys > phys))
+			data_offset = phys - pgmap_phys;
+		pr_debug("%s: offset detected phys=%llx pgmap_phys=%llx offset=%llx shift=%llx\n",
+		       __func__, phys, pgmap_phys, data_offset, vmemmap_shift);
+	}
+	dev_dax->virt_addr = addr + data_offset;
 	inode = dax_inode(dax_dev);
 	cdev = inode->i_cdev;
 	cdev_init(cdev, &dax_fops);

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