Re: [PATCH v3 04/11] ext2: remove support for DAX PMD faults

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

On Wed, Sep 28, 2016 at 07:47:20AM +1000, Dave Chinner wrote:
> On Tue, Sep 27, 2016 at 02:47:55PM -0600, Ross Zwisler wrote:
> > DAX PMD support was added via the following commit:
> > 
> > commit e7b1ea2ad658 ("ext2: huge page fault support")
> > 
> > I believe this path to be untested as ext2 doesn't reliably provide block
> > allocations that are aligned to 2MiB.  In my testing I've been unable to
> > get ext2 to actually fault in a PMD.  It always fails with a "pfn
> > unaligned" message because the sector returned by ext2_get_block() isn't
> > aligned.
> > 
> > I've tried various settings for the "stride" and "stripe_width" extended
> > options to mkfs.ext2, without any luck.
> > 
> > Since we can't reliably get PMDs, remove support so that we don't have an
> > untested code path that we may someday traverse when we happen to get an
> > aligned block allocation.  This should also make 4k DAX faults in ext2 a
> > bit faster since they will no longer have to call the PMD fault handler
> > only to get a response of VM_FAULT_FALLBACK.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> ....
> > @@ -154,7 +133,6 @@ static int ext2_dax_pfn_mkwrite(struct vm_area_struct *vma,
> >  
> >  static const struct vm_operations_struct ext2_dax_vm_ops = {
> >  	.fault		= ext2_dax_fault,
> > -	.pmd_fault	= ext2_dax_pmd_fault,
> >  	.page_mkwrite	= ext2_dax_fault,
> >  	.pfn_mkwrite	= ext2_dax_pfn_mkwrite,
> >  };
> 
> Would it be better to put a comment mentioning this here? So as the
> years go by, this reminds people not to bother trying to implement
> it?
> 
> /*
>  * .pmd_fault is not supported for DAX because allocation in ext2
>  * cannot be reliably aligned to huge page sizes and so pmd faults
>  * will always fail and fail back to regular faults.
>  */

Sure, this seems like a good idea.  I'll add it, thanks.