Re: [PATCH V7 03/19] dax: add fsdev.c driver for fs-dax on character dax

Ira Weiny
2026-02-13T21:02:21Z

John Groves wrote:
> From: John Groves <john@groves.net>
>
> The new fsdev driver provides pages/folios initialized compatibly with
> fsdax - normal rather than devdax-style refcounting, and starting out
> with order-0 folios.
>
> When fsdev binds to a daxdev, it is usually (always?) switching from the
> devdax mode (device.c), which pre-initializes compound folios according
> to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> folios into a fsdax-compatible state.
>
> A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> mmap capability.
>
> In this commit is just the framework, which remaps pages/folios compatibly
> with fsdax.
>
> Enabling dax changes:
>
> - bus.h: add DAXDRV_FSDEV_TYPE driver type
> - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> - dax.h: prototype inode_dax(), which fsdev needs
>
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Suggested-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: John Groves <john@groves.net>
> ---
> MAINTAINERS | 8 ++
> drivers/dax/Makefile | 6 ++
> drivers/dax/bus.c | 4 +
> drivers/dax/bus.h | 1 +
> drivers/dax/fsdev.c | 242 +++++++++++++++++++++++++++++++++++++++++++
> fs/dax.c | 1 +
> include/linux/dax.h | 5 +
> 7 files changed, 267 insertions(+)
> create mode 100644 drivers/dax/fsdev.c
>

[snip]

> +
> +static int fsdev_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;
> + int rc, i;
> +
> + if (static_dev_dax(dev_dax)) {
> + if (dev_dax->nr_range > 1) {
> + dev_warn(dev, "static pgmap / multi-range device conflict\n");
> + return -EINVAL;
> + }
> +
> + pgmap = dev_dax->pgmap;
> + } else {
> + size_t pgmap_size;
> +
> + if (dev_dax->pgmap) {
> + dev_warn(dev, "dynamic-dax with pre-populated page map\n");
> + return -EINVAL;
> + }
> +
> + pgmap_size = struct_size(pgmap, ranges, dev_dax->nr_range - 1);
> + pgmap = devm_kzalloc(dev, pgmap_size, GFP_KERNEL);
> + if (!pgmap)
> + return -ENOMEM;
> +
> + pgmap->nr_range = dev_dax->nr_range;
> + dev_dax->pgmap = pgmap;
> +
> + for (i = 0; i < dev_dax->nr_range; i++) {
> + struct range *range = &dev_dax->ranges[i].range;
> +
> + pgmap->ranges[i] = *range;
> + }
> + }
> +
> + for (i = 0; i < dev_dax->nr_range; i++) {
> + struct range *range = &dev_dax->ranges[i].range;
> +
> + if (!devm_request_mem_region(dev, range->start,
> + range_len(range), dev_name(dev))) {
> + dev_warn(dev, "mapping%d: %#llx-%#llx could not reserve range\n",
> + i, range->start, range->end);
> + return -EBUSY;
> + }
> + }

All of the above code is AFAICT exactly the same as the dev_dax driver.
Isn't there a way to make this common?

The rest of the common code is simple enough.

> +
> + /*
> + * FS-DAX compatible mode: Use MEMORY_DEVICE_FS_DAX type and
> + * do NOT set vmemmap_shift. This leaves folios at order-0,
> + * allowing fs-dax to dynamically create compound folios as needed
> + * (similar to pmem behavior).
> + */
> + pgmap->type = MEMORY_DEVICE_FS_DAX;
> + pgmap->ops = &fsdev_pagemap_ops;
> + pgmap->owner = dev_dax;
> +
> + /*
> + * CRITICAL DIFFERENCE from device.c:
> + * We do NOT set vmemmap_shift here, even if align > PAGE_SIZE.
> + * This ensures folios remain order-0 and are compatible with
> + * fs-dax's folio management.
> + */
> +
> + addr = devm_memremap_pages(dev, pgmap);
> + if (IS_ERR(addr))
> + return PTR_ERR(addr);
> +
> + /*
> + * Clear any stale compound folio state left over from a previous
> + * driver (e.g., device_dax with vmemmap_shift).
> + */
> + fsdev_clear_folio_state(dev_dax);
> +
> + /* 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;
> +
> + if (!WARN_ON(pgmap_phys > phys))
> + data_offset = phys - pgmap_phys;
> +
> + pr_debug("%s: offset detected phys=%llx pgmap_phys=%llx offset=%llx\n",
> + __func__, phys, pgmap_phys, data_offset);
> + }
> +
> + inode = dax_inode(dax_dev);
> + cdev = inode->i_cdev;
> + cdev_init(cdev, &fsdev_fops);
> + cdev->owner = dev->driver->owner;
> + cdev_set_parent(cdev, &dev->kobj);
> + rc = cdev_add(cdev, dev->devt, 1);
> + if (rc)
> + return rc;
> +
> + rc = devm_add_action_or_reset(dev, fsdev_cdev_del, cdev);
> + if (rc)
> + return rc;
> +
> + run_dax(dax_dev);
> + return devm_add_action_or_reset(dev, fsdev_kill, dev_dax);
> +}
> +

[snip]

> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 9d624f4d9df6..fe1315135fdd 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -51,6 +51,10 @@ struct dax_holder_operations {
>
> #if IS_ENABLED(CONFIG_DAX)
> struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
> +
> +#if IS_ENABLED(CONFIG_DEV_DAX_FS)
> +struct dax_device *inode_dax(struct inode *inode);
> +#endif

I don't understand why this hunk is added here but then removed in a later
patch? Why can't this be placed below? ...

> void *dax_holder(struct dax_device *dax_dev);
> void put_dax(struct dax_device *dax_dev);
> void kill_dax(struct dax_device *dax_dev);
> @@ -153,6 +157,7 @@ static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
> #if IS_ENABLED(CONFIG_FS_DAX)
> int dax_writeback_mapping_range(struct address_space *mapping,
> struct dax_device *dax_dev, struct writeback_control *wbc);
> +int dax_folio_reset_order(struct folio *folio);

... Here?

Ira

[snip]