Re: [PATCH V9 5/8] dax: Add dax_operations for use by fs-dax on fsdev dax

[Dave Jiang <dave.jiang@intel.com>]

On 3/23/26 5:39 PM, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> fsdev: Add dax_operations for use by famfs.
> 
> This replicates the functionality from drivers/nvdimm/pmem.c that
> conventional fs-dax file systems (e.g. xfs) use to support dax
> read/write/mmap to a daxdev - without which famfs can't sit atop a
> daxdev.
> 
> - These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> - fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
>   newly stored as dev_dax->virt_addr by dev_dax_probe().
> - The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
>   for read/write (dax_iomap_rw())
> - fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
>   tested yet. I'm looking for suggestions as to how to test those.
> - dax-private.h: add dev_dax->cached_size, which fsdev needs to
>   remember. The dev_dax size cannot change while a driver is bound
>   (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
>   at probe time allows fsdev's direct_access path can use it without
>   acquiring dax_dev_rwsem (which isn't exported anyway).
> 
> Signed-off-by: John Groves <john@groves.net>
> ---
>  drivers/dax/dax-private.h |  1 +
>  drivers/dax/fsdev.c       | 84 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 85 insertions(+)
> 
> diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
> index 7a3727d76a68..ee8f3af8387f 100644
> --- a/drivers/dax/dax-private.h
> +++ b/drivers/dax/dax-private.h
> @@ -85,6 +85,7 @@ struct dev_dax {
>  	struct dax_region *region;
>  	struct dax_device *dax_dev;
>  	void *virt_addr;
> +	u64 cached_size;

Just caught this. Need a kdoc entry.

DJ

>  	unsigned int align;
>  	int target_node;
>  	bool dyn_id;
> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> index c75478d3d548..be3d2b0e8418 100644
> --- a/drivers/dax/fsdev.c
> +++ b/drivers/dax/fsdev.c
> @@ -28,6 +28,85 @@
>   * - No mmap support - all access is through fs-dax/iomap
>   */
>  
> +static void fsdev_write_dax(void *pmem_addr, struct page *page,
> +		unsigned int off, unsigned int len)
> +{
> +	while (len) {
> +		void *mem = kmap_local_page(page);
> +		unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
> +
> +		memcpy_flushcache(pmem_addr, mem + off, chunk);
> +		kunmap_local(mem);
> +		len -= chunk;
> +		off = 0;
> +		page++;
> +		pmem_addr += chunk;
> +	}
> +}
> +
> +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> +			long nr_pages, enum dax_access_mode mode, void **kaddr,
> +			unsigned long *pfn)
> +{
> +	struct dev_dax *dev_dax = dax_get_private(dax_dev);
> +	size_t size = nr_pages << PAGE_SHIFT;
> +	size_t offset = pgoff << PAGE_SHIFT;
> +	void *virt_addr = dev_dax->virt_addr + offset;
> +	phys_addr_t phys;
> +	unsigned long local_pfn;
> +
> +	phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
> +	if (phys == -1) {
> +		dev_dbg(&dev_dax->dev,
> +			"pgoff (%#lx) out of range\n", pgoff);
> +		return -EFAULT;
> +	}
> +
> +	if (kaddr)
> +		*kaddr = virt_addr;
> +
> +	local_pfn = PHYS_PFN(phys);
> +	if (pfn)
> +		*pfn = local_pfn;
> +
> +	/*
> +	 * Use cached_size which was computed at probe time. The size cannot
> +	 * change while the driver is bound (resize returns -EBUSY).
> +	 */
> +	return PHYS_PFN(min(size, dev_dax->cached_size - offset));
> +}
> +
> +static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
> +			pgoff_t pgoff, size_t nr_pages)
> +{
> +	void *kaddr;
> +
> +	WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
> +	__fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
> +	fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
> +	return 0;
> +}
> +
> +static long fsdev_dax_direct_access(struct dax_device *dax_dev,
> +		  pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
> +		  void **kaddr, unsigned long *pfn)
> +{
> +	return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
> +					 kaddr, pfn);
> +}
> +
> +static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
> +		void *addr, size_t bytes, struct iov_iter *i)
> +{
> +	return _copy_from_iter_flushcache(addr, bytes, i);
> +}
> +
> +static const struct dax_operations dev_dax_ops = {
> +	.direct_access = fsdev_dax_direct_access,
> +	.zero_page_range = fsdev_dax_zero_page_range,
> +	.recovery_write = fsdev_dax_recovery_write,
> +};
> +
>  static void fsdev_cdev_del(void *cdev)
>  {
>  	cdev_del(cdev);
> @@ -167,6 +246,11 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
>  		}
>  	}
>  
> +	/* Cache size now; it cannot change while driver is bound */
> +	dev_dax->cached_size = 0;
> +	for (i = 0; i < dev_dax->nr_range; i++)
> +		dev_dax->cached_size += range_len(&dev_dax->ranges[i].range);
> +
>  	/*
>  	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
>  	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this