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    SubjectRe: [RFC PATCH 04/20] dev_dax_iomap: Save the kva from memremap
    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);


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