Re: [PATCH 17/19] mlock, thp: HACK: split all pages in VM_LOCKED vma

[Naoya Horiguchi]

On Wed, Nov 05, 2014 at 04:49:52PM +0200, Kirill A. Shutemov wrote:
> We don't yet handle mlocked pages properly with new THP refcounting.
> For now we split all pages in VMA on mlock and disallow khugepaged
> collapse pages in the VMA. If split failed on mlock() we fail the
> syscall with -EBUSY.
> ---
...

> @@ -542,6 +530,60 @@ next:
>  	}
>  }
>  
> +static int thp_split(pmd_t *pmd, unsigned long addr, unsigned long end,
> +		struct mm_walk *walk)
> +{
> +	spinlock_t *ptl;
> +	struct page *page = NULL;
> +	pte_t *pte;
> +	int err = 0;
> +
> +retry:
> +	if (pmd_none(*pmd))
> +		return 0;
> +	if (pmd_trans_huge(*pmd)) {
> +		if (is_huge_zero_pmd(*pmd)) {
> +			split_huge_pmd(walk->vma, pmd, addr);
> +			return 0;
> +		}
> +		ptl = pmd_lock(walk->mm, pmd);
> +		if (!pmd_trans_huge(*pmd)) {
> +			spin_unlock(ptl);
> +			goto retry;
> +		}
> +		page = pmd_page(*pmd);
> +		VM_BUG_ON_PAGE(!PageHead(page), page);
> +		get_page(page);
> +		spin_unlock(ptl);
> +		err = split_huge_page(page);
> +		put_page(page);
> +		return err;
> +	}
> +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> +	do {
> +		if (!pte_present(*pte))
> +			continue;
> +		page = vm_normal_page(walk->vma, addr, *pte);
> +		if (!page)
> +			continue;
> +		if (PageTransCompound(page)) {
> +			page = compound_head(page);
> +			get_page(page);
> +			spin_unlock(ptl);
> +			err = split_huge_page(page);
> +			spin_lock(ptl);
> +			put_page(page);
> +			if (!err) {
> +				VM_BUG_ON_PAGE(compound_mapcount(page), page);
> +				VM_BUG_ON_PAGE(PageTransCompound(page), page);

If split_huge_page() succeeded, we don't have to continue the iteration,
so break this loop here?

Thanks,
Naoya Horiguchi

> +			} else
> +				break;
> +		}
> +	} while (pte++, addr += PAGE_SIZE, addr != end);
> +	pte_unmap_unlock(pte - 1, ptl);
> +	return err;
> +}
> +
>  /*
>   * mlock_fixup  - handle mlock[all]/munlock[all] requests.
>   *--
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