Re: [PATCH V3 04/21] dax: Add dax_operations for use by fs-dax on fsdev dax

[John Groves]

On 26/01/08 11:50AM, Jonathan Cameron wrote:
> On Wed,  7 Jan 2026 09:33:13 -0600
> John Groves <John@xxxxxxxxxx> wrote:
> 
> > From: John Groves <John@xxxxxxxxxx>
> > 
> Hi John
> 
> The description should generally make sense without the title.
> Sometimes that means more or less repeating the title.
> 
> A few other things inline.

Will do

> 
> > * 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@xxxxxxxxxx>
> 
> > diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> > index c5c660b193e5..9e2f83aa2584 100644
> > --- a/drivers/dax/fsdev.c
> > +++ b/drivers/dax/fsdev.c
> > @@ -27,6 +27,81 @@
> >   * - 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);
> 
> I guess it's pretty simple, but do we care about HIGHMEM for this
> new feature?  Maybe it's just easier to support it than argue about it however ;)

I think this compiles to zero overhead, and is an established pattern -
but I'm ok following a consensus elsewhere...

> 
> > +		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);
> 
> Use size given you already computed it.

Not sure I follow. nr_pages is the size of the access or fault, not the size
of the device. 

> 
> > +
> > +	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_t(size_t, size, dev_dax->cached_size - offset));
> 
> Is the min_t() needed?  min() is pretty good at picking right types these days.

Changed to min()

> 
> > +}
> > +
> > +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);
> 
> Alignment in this file is a bit random, but I'd at least align this one
> after the (

Done, thanks!

John