Re: dax alignment problem on arm64 (and other achitectures)

From: Dan Williams <dan.j.williams@intel.com>
To: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: David Hildenbrand <david@redhat.com>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Sasha Levin <sashal@kernel.org>,
	Tyler Hicks <tyhicks@linux.microsoft.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	Oscar Salvador <osalvador@suse.de>,
	Vlastimil Babka <vbabka@suse.cz>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Marc Zyngier <maz@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	James Morse <james.morse@arm.com>,
	James Morris <jmorris@namei.org>
Subject: Re: dax alignment problem on arm64 (and other achitectures)
Date: Fri, 29 Jan 2021 12:26:07 -0800	[thread overview]
Message-ID: <CAPcyv4jLmDvQ+e7QSQjOsAccSnhpvm9J5kHsA1OCXcaOm7BrMA@mail.gmail.com> (raw)
In-Reply-To: <CA+CK2bBJmnTVF8ZfwLyLqgjgo63G-rVQTYwUqgmx8wXFtRH9-g@mail.gmail.com>

On Fri, Jan 29, 2021 at 5:51 AM Pavel Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> > Since we last talked about this the enabling for EFI "Special Purpose"
> > / Soft Reserved Memory has gone upstream and instantiates device-dax
> > instances for address ranges marked with EFI_MEMORY_SP attribute.
> > Critically this way of declaring device-dax removes the consideration
> > of it as persistent memory and as such no metadata reservation. So, if
> > you are willing to maintain the metadata external to the device (which
> > seems reasonable for your environment) and have your platform firmware
> > / kernel command line mark it as EFI_CONVENTIONAL_MEMORY +
> > EFI_MEMORY_SP, then these reserve-free dax-devices will surface.
>
> Hi Dan,
>
> This is cool. Does it allow conversion between devdax and fsdax so DAX
> aware filesystem can be installed and data can be put there to be
> preserved across the reboot?
>

It does not because it's not "pmem" by this designation.

Instead if you want fsdax, zero metadata on the device, and the
ability to switch from fsdax to devdax I think that could be achieved
with a new sysfs attribute at the region-device level. Currently the
mode of a namespace with no metadata on it defaults to "raw" mode
where "raw" treats the pmem as a persistent memory block device with
no DAX capability. There's no reason the default could instead be
devdax with pages mapped.

Something like:
ndctl disable-region region0
echo 1 > /sys/bus/nd/devices/region0/pagemap
echo devdax > /sys/bus/nd/devices/region0/raw_default
ndctl enable-region region0

...where the new pagemap attribute does set_bit(ND_REGION_PAGEMAP,
&nd_region->flags), and raw_default arranges for the namespace to be
shunted over to devdax.