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

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Message-ID: <20260108161013.00001916@huawei.com>
Date: Thu, 8 Jan 2026 16:10:13 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: John Groves <John@...ves.net>
CC: Miklos Szeredi <miklos@...redi.hu>, Dan Williams
	<dan.j.williams@...el.com>, Bernd Schubert <bschubert@....com>, "Alison
 Schofield" <alison.schofield@...el.com>, John Groves <jgroves@...ron.com>,
	Jonathan Corbet <corbet@....net>, Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>, Matthew Wilcox <willy@...radead.org>, Jan
 Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>, "David
 Hildenbrand" <david@...nel.org>, Christian Brauner <brauner@...nel.org>,
	"Darrick J . Wong" <djwong@...nel.org>, Randy Dunlap <rdunlap@...radead.org>,
	Jeff Layton <jlayton@...nel.org>, Amir Goldstein <amir73il@...il.com>, Stefan
 Hajnoczi <shajnocz@...hat.com>, Joanne Koong <joannelkoong@...il.com>, Josef
 Bacik <josef@...icpanda.com>, Bagas Sanjaya <bagasdotme@...il.com>, Chen
 Linxuan <chenlinxuan@...ontech.com>, "James Morse" <james.morse@....com>,
	Fuad Tabba <tabba@...gle.com>, "Sean Christopherson" <seanjc@...gle.com>,
	Shivank Garg <shivankg@....com>, Ackerley Tng <ackerleytng@...gle.com>,
	Gregory Price <gourry@...rry.net>, Aravind Ramesh <arramesh@...ron.com>, Ajay
 Joshi <ajayjoshi@...ron.com>, <venkataravis@...ron.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <linux-cxl@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V3 04/21] dax: Add dax_operations for use by fs-dax on
 fsdev dax

On Thu, 8 Jan 2026 09:59:08 -0600
John Groves <John@...ves.net> wrote:

> On 26/01/08 11:50AM, Jonathan Cameron wrote:
> > On Wed,  7 Jan 2026 09:33:13 -0600
> > John Groves <John@...ves.net> wrote:
> >   
> > > From: John Groves <John@...ves.net>
> > >   
> > 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@...ves.net>  
> >   
> > > 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...

That's fair, probably just keep it.

> > > +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. 

Just above:

size_t size = nr_pages << PAGE_SHIFT;

Jonathan