Re: [PATCH v3] Fix ext4 fault handling when mounted with -o dax,ro

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Wed, Aug 23, 2017 at 02:26:52PM -0700, rdodgen-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org wrote:
> From: Randy Dodgen <dodgen-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> 
> If an ext4 filesystem is mounted with both the DAX and read-only
> options, executables on that filesystem will fail to start (claiming
> 'Segmentation fault') due to the fault handler returning
> VM_FAULT_SIGBUS.
> 
> This is due to the DAX fault handler (see ext4_dax_huge_fault)
> attempting to write to the journal when FAULT_FLAG_WRITE is set. This is
> the wrong behavior for write faults which will lead to a COW page; in
> particular, this fails for readonly mounts.
> 
> This change avoids journal writes for faults that are expected to COW.
> 
> It might be the case that this could be better handled in
> ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside
> dax_iomap_fault). These is some overlap already (e.g. grabbing journal
> handles).
> 
> Signed-off-by: Randy Dodgen <dodgen-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

Cool, looks good from the DAX point of view.  You can add:

Reviewed-by: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>