Re: FAILED: patch "[PATCH] dm: prevent DAX mounts if not supported" failed to apply to 4.9-stable tree

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Sun, Jul 01, 2018 at 04:25:45PM +0200, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 4.9-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.
> 
> thanks,
> 
> greg k-h
> 
> ------------------ original commit in Linus's tree ------------------
> 
> From dbc626597c39b24cefce09fbd8e9dea85869a801 Mon Sep 17 00:00:00 2001
> From: Ross Zwisler <ross.zwisler@linux.intel.com>
> Date: Tue, 26 Jun 2018 16:30:41 -0600
> Subject: [PATCH] dm: prevent DAX mounts if not supported
> 
> Currently device_supports_dax() just checks to see if the QUEUE_FLAG_DAX
> flag is set on the device's request queue to decide whether or not the
> device supports filesystem DAX.  Really we should be using
> bdev_dax_supported() like filesystems do at mount time.  This performs
> other tests like checking to make sure the dax_direct_access() path works.
> 
> We also explicitly clear QUEUE_FLAG_DAX on the DM device's request queue if
> any of the underlying devices do not support DAX.  This makes the handling
> of QUEUE_FLAG_DAX consistent with the setting/clearing of most other flags
> in dm_table_set_restrictions().
> 
> Now that bdev_dax_supported() explicitly checks for QUEUE_FLAG_DAX, this
> will ensure that filesystems built upon DM devices will only be able to
> mount with DAX if all underlying devices also support DAX.

Hey Greg,

It turns out that this series isn't needed for the v4.9-stable tree.  The
outcome we want with this series is to prevent DM devices made of one
DAX-capable namespace and one DAX-incapable block device from using DAX.

We needed patches to prevent this from happening in v4.14-stable and
v4.17-stable, but v4.9-stable already does the correct thing.

For those interested in the details, the DM device doesn't have QUEUE_FLAG_DAX
set per the check in dm_setup_md_queue(), and we notice that in
bdev_direct_access() via the blk_queue_dax() check.  This causes the
filesystem to see that DAX isn't supported.

So, it looks like the bug was introduced when that check was removed from
bdev_direct_access(), not back when DAX support was initially introduced in DM
like I thought previously.

Thanks,
- Ross