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

[John Groves <John@groves.net>]

On 26/03/25 03:40PM, Dave Jiang wrote:
> 
> 
> 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>
> 
> Couple nits below while I'm stealing code from you.

:D

> 
> > ---
> >  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;
> >  	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,
> 
> addr instead of pmem_addr? copy pasta error?

Yep, fixed thanks

> 
> > +		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);
> 
> you can use 'size' instead here since it's previously computed already.
> 
> DJ

Indeed - thanks!

John