Re: [PATCH] dax: Split pmd map when fallback on COW

[Dan Williams]

On Mon, Nov 23, 2015 at 12:05 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> An infinite loop of PMD faults was observed when attempted to
> mlock() a private read-only PMD mmap'd range of a DAX file.
>
> __dax_pmd_fault() simply returns with VM_FAULT_FALLBACK when
> falling back to PTE on COW.  However, __handle_mm_fault()
> returns without falling back to handle_pte_fault() because
> a PMD map is present in this case.
>
> Change __dax_pmd_fault() to split the PMD map, if present,
> before returning with VM_FAULT_FALLBACK.
>
> Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
>  fs/dax.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 43671b6..3405583 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -546,8 +546,10 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
>                 return VM_FAULT_FALLBACK;
>
>         /* Fall back to PTEs if we're going to COW */
> -       if (write && !(vma->vm_flags & VM_SHARED))
> +       if (write && !(vma->vm_flags & VM_SHARED)) {
> +               split_huge_page_pmd(vma, address, pmd);
>                 return VM_FAULT_FALLBACK;
> +       }

Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>

I took a closer look at dax's CONFIG_TRANSPARENT_HUGEPAGE interactions
and it turns out THP is a performance enhancement not a functional
dependency.  I.e. a performance enhancement to use a huge_zero_page
where available, but not a requirement.

I'll fold this in with my series make pmd_trans_huge() return false
for non-huge_zero_page dax mappings, and in that case I'll need to
up-level the call to  pmdp_huge_clear_flush_notify() from
__split_huge_page_pmd.
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