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@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
> 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.
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