Re: [PATCH 3/3] mm, thp: Do not loose dirty bit in __split_huge_pmd_locked()

[Martin Schwidefsky]

On Wed, 14 Jun 2017 16:51:43 +0300
"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:

> Until pmdp_invalidate() pmd entry is present and CPU can update it,
> setting dirty. Currently, we tranfer dirty bit to page too early and
> there is window when we can miss dirty bit.
> 
> Let's call SetPageDirty() after pmdp_invalidate().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ...
> @@ -2046,6 +2043,14 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	 * pmd_populate.
>  	 */
>  	pmdp_invalidate(vma, haddr, pmd);
> +
> +	/*
> +	 * Transfer dirty bit to page after pmd invalidated, so CPU would not
> +	 * be able to set it under us.
> +	 */
> +	if (pmd_dirty(*pmd))
> +		SetPageDirty(page);
> +
>  	pmd_populate(mm, pmd, pgtable);
> 
>  	if (freeze) {

That won't work on s390. After pmdp_invalidate the pmd entry is gone,
it has been replaced with _SEGMENT_ENTRY_EMPTY. This includes the
dirty and referenced bits. The old scheme is

        entry = *pmd;
        pmdp_invalidate(vma, addr, pmd);
	if (pmd_dirty(entry))
		...

Could we change pmdp_invalidate to make it return the old pmd entry?
The pmdp_xchg_direct function already returns it, for s390 that would
be an easy change. The above code snippet would change like this:

	entry = pmdp_invalidate(vma, addr, pmd);
	if (pmd_dirty(entry))
		...

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.