Re: [PATCH V7 05/19] dax: Add dax_operations for use by fs-dax on fsdev dax

Ira Weiny
2026-02-14T16:07:00Z

John Groves wrote:
> From: John Groves <John@Groves.net>
>
> fsdev: Add dax_operations for use by famfs
>
> - 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())

I thought this driver did not support mmap?

> - 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).
>

None of the above explains exactly why this code is needed. Rather it
just explains what it does.

I'm not 100% clear on why this is needed in the driver and why this is not
a layering violation which is going to bite us later?

Ira

> Signed-off-by: John Groves <john@groves.net>
> ---
> drivers/dax/dax-private.h | 1 +
> drivers/dax/fsdev.c | 85 +++++++++++++++++++++++++++++++++++++++
> 2 files changed, 86 insertions(+)
>
> diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
> index 4ae4d829d3ee..092f4ae024ea 100644
> --- a/drivers/dax/dax-private.h
> +++ b/drivers/dax/dax-private.h
> @@ -83,6 +83,7 @@ struct dev_dax {
> struct dax_region *region;
> struct dax_device *dax_dev;
> void *virt_addr;
> + u64 cached_size;
> unsigned int align;
> int target_node;
> bool dyn_id;
> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> index 72f78f606e06..5d17ad39227f 100644
> --- a/drivers/dax/fsdev.c
> +++ b/drivers/dax/fsdev.c
> @@ -28,6 +28,86 @@
> * - 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;
> +
> + WARN_ON(!dev_dax->virt_addr);
> +
> + 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 -ERANGE;
> + }
> +
> + 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)
> {
> @@ -163,6 +243,11 @@ 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);
> +
> /*
> * FS-DAX compatible mode: Use MEMORY_DEVICE_FS_DAX type and
> * do NOT set vmemmap_shift. This leaves folios at order-0,
> --
> 2.52.0
>
>
>