Re: can we finally kill off CONFIG_FS_DAX_LIMITED

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Joao Martins <joao.m.martins@oracle.com>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Christoph Hellwig <hch@lst.de>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Linux NVDIMM <nvdimm@lists.linux.dev>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	Alex Sierra <alex.sierra@amd.com>,
	"Kuehling, Felix" <Felix.Kuehling@amd.com>,
	Linux MM <linux-mm@kvack.org>,
	Ralph Campbell <rcampbell@nvidia.com>,
	Alistair Popple <apopple@nvidia.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>
Subject: Re: can we finally kill off CONFIG_FS_DAX_LIMITED
Date: Tue, 19 Oct 2021 14:54:08 -0300	[thread overview]
Message-ID: <20211019175408.GJ3686969@ziepe.ca> (raw)
In-Reply-To: <CAPcyv4jAQVSKB7rts5Mfu0JRtB-b1NGFgu03+8-ja8o11d1vQA@mail.gmail.com>

On Tue, Oct 19, 2021 at 10:38:42AM -0700, Dan Williams wrote:

> > So we can just delete the detection of the page size and rely on the
> > zap code to wipe out the entire level, not split it. Which is what we
> > have today already.
> 
> As Joao points out, userspace wants to know the blast radius of the
> unmap for historical reasons. I do think it's worth deprecating that
> somehow... providing a better error management interface is part of
> the DAX-reflink enabling.

OK, it makes sense.

I have a less invasive idea though - emulate what zap is doing:

      if (!pud_present(*pud))
               return 0;
      if (pud_leaf(*pud))
             return PUD_SHIFT;

      if (!pmd_present(*pud))
               return 0;
      if (pmd_leaf(*pud))
             return PMD_SHIFT;
      return PAGE_SHIFT;

Which would return the "blast radius" of the unmap_mapping_range()
when it rounds up to the left page level that contains the VA.

Now it doesn't need the pte_devmap test..

And when both DAX's learn to use compound_head this can be deleted.

Jason