Re: [PATCH] dax: adding fsync/msync support for device DAX

From: Dan Williams <dan.j.williams@intel.com>
To: Jeff Moyer <jmoyer@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH] dax: adding fsync/msync support for device DAX
Date: Wed, 11 Apr 2018 09:27:22 -0700	[thread overview]
Message-ID: <CAPcyv4guech76UWc7xo+SAzKOv8dT83HTUoUPVUCx5Kjuxy5Vg@mail.gmail.com> (raw)
In-Reply-To: <x49d0z5pw13.fsf@segfault.boston.devel.redhat.com>

On Wed, Apr 11, 2018 at 9:06 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
> Christoph Hellwig <hch@infradead.org> writes:
>
>> On Fri, Apr 06, 2018 at 03:41:39PM -0700, Dan Williams wrote:
>>> Yes, but the trust interface definition is what is missing, especially
>>> when we consider memmap=ss!nn and qemu-kvm. For example do we turn off
>>> DAX and/or MAP_SYNC on all platforms that don't provide a positive "I
>>> have ADR" indication (ACPI 6.2 Section 5.2.25.9 NFIT Platform
>>> Capabilities Structure)?
>>
>> Sounds like a default.
>
> Which do you turn off?  DAX or MAP_SYNC (or both)?

Only MAP_SYNC I would say, because then userspace can't opt out of
fsync/msync and it gives us a chance to "Do The Right Thing (TM)" with
respect to WPQ flush.

> Systems that support ACPI versions earlier than 6.2 could provide
> flush hint addresses.  In that case, we could assume no ADR, and turn
> off MAP_SYNC, but still allow DAX.  Note that I'm not aware of any
> hardware that actually falls into this category.
>
> Platforms prior to 6.2 that don't support flush hints are currently
> assumed to implement ADR.  This proposal would change that default.
>
>>> Require opt-in when the user has trust in the hardware config that
>>> the kernel can't verify?
>>
>> And that sounds like a good config nob ontop of that default.
>
> Well, we could also make a white-list for known good platforms.  I
> assume HPE's systems would be on that list.  Otherwise we'd have to cook
> up udev rules, I guess?

udev rules sound good. We can distribute them in the ndctl package. I
think I would handle this by making
/sys/bus/nd/devices/regionX/persistence_domain writable to opt-in to a
user specified persistence_domain. If the persistence_domain attribute
file is not writable then you know you're on a kernel that blindly
trusts all pmem descriptions as MAP_SYNC capable.

> Dan, is this something you're working on now?

No, it's behind finalizing dax-dma-vs-truncate and memcpy_mcsafe for
user copies in my queue...
_______________________________________________
Linux-nvdimm mailing list
Linux-nvdimm@lists.01.org
https://lists.01.org/mailman/listinfo/linux-nvdimm