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

John Groves
2026-02-17T17:56:43Z

On 26/02/13 03:05PM, Ira Weiny wrote:
> 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.

dev_dax_probe() and fsdev_dax_probe() do indeed have some "same code" -
range validity checking and pgmap setup, from the top of probe through
the for loop above. After that they're different. Also, I just did a scan
and the probe function seems like the only remaining common code between
device.c and fsdev.c.

These are separate kmods; that code could certainly be factored out and
shared, but it would need to go somewhere common (maybe bus.c)?

So both device.c and fsdev.c would call bus.c:dax_prepare_pgmap() or
some such.

I feel like this might not be worth factoring out, but I'm happy to do it
if you and/or the dax team prefer it factored out and shared.

>
> > +
> > + /*
> > + * 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?

Done, thanks - good catch. That was just sloppy factoring into a series on
my part.

>
> Ira
>
> [snip]

Thanks for the reviewing Ira!

Regards,
John