Re: [RFC PATCH 04/20] dev_dax_iomap: Save the kva from memremap

Jonathan Cameron
2024-02-26T12:21:43Z

On Fri, 23 Feb 2024 11:41:48 -0600
John Groves <John@Groves.net> wrote:

> 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 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.

Probably good to have info on when this happens and preferably why
this dragon is there.

>
> Dragons...
>
> Signed-off-by: John Groves <john@groves.net>
> ---
> 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..894eb1c66b4a 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;
> + u64 virt_addr;

Why as a u64? If it's a virt address why not just void *?

> unsigned int align;
> int target_node;
> bool dyn_id;
> diff --git a/drivers/dax/device.c b/drivers/dax/device.c
> index 40ba660013cf..6cd79d00fe1b 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 = (u64)dev_dax->ranges[0].range.start;

Why the cast? Ranges use u64s internally.

> + u64 pgmap_phys = (u64)dev_dax->pgmap[0].range.start;
> + u64 vmemmap_shift = (u64)dev_dax->pgmap[0].vmemmap_shift;
> +
> + if (!WARN_ON(pgmap_phys > phys))
> + data_offset = phys - pgmap_phys;
> +
> + pr_notice("%s: offset detected phys=%llx pgmap_phys=%llx offset=%llx shift=%llx\n",
> + __func__, phys, pgmap_phys, data_offset, vmemmap_shift);

pr_debug() + dynamic debug will then deal with __func__ for you.

> + }
> + dev_dax->virt_addr = (u64)addr + data_offset;
> +
> inode = dax_inode(dax_dev);
> cdev = inode->i_cdev;
> cdev_init(cdev, &dax_fops);