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

From: Dave Chinner
Date: Tue Sep 27 2016 - 17:47:50 EST

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@xxxxxxxxxxxxxxx>
> @@ -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

* .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.

Dave Chinner