Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 27 May 2010 13:43:00 -0700 Doug Doan <dougd@cray.com> wrote:

> 
> When a copy-on-write occurs, we take one of two paths in handle_mm_fault: 
> through handle_pte_fault for normal pages, or through hugetlb_fault for huge pages.
> 
> In the normal page case, we eventually get to do_wp_page and call mmu notifiers 
> via ptep_clear_flush_notify. There is no callout to the mmmu notifiers in the 
> huge page case. This patch fixes that.
> 
> Signed-off-by: Doug Doan <dougd@cray.com>
> ---
> 
> [patch  text/plain (802B)]
> --- mm/hugetlb.c.orig	2010-05-27 13:07:58.569546314 -0700
> +++ mm/hugetlb.c	2010-05-26 14:41:06.449296524 -0700

(In patch -p1 form, please.  So a/mm/hugetlb.c)

> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
>  	ptep = huge_pte_offset(mm, address & huge_page_mask(h));
>  	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
>  		/* Break COW */
> +		mmu_notifier_invalidate_range_start(mm,
> +			address & huge_page_mask(h),
> +			(address & huge_page_mask(h)) + huge_page_size(h));
>  		huge_ptep_clear_flush(vma, address, ptep);
>  		set_huge_pte_at(mm, address, ptep,
>  				make_huge_pte(vma, new_page, 1));
>  		/* Make the old page be freed below */
>  		new_page = old_page;
> +		mmu_notifier_invalidate_range_end(mm,
> +			address & huge_page_mask(h),
> +			(address & huge_page_mask(h)) + huge_page_size(h));
>  	}
>  	page_cache_release(new_page);
>  	page_cache_release(old_page);

This causes mmu_notifier_invalidate_range_start() to be called under
page_table_lock.  The immediately preceding code seems to take some
care to avoid doing that.  I took a quick look at other callsites and
cannot immediately see other cases where
mmu_notifier_invalidate_range_start/end() are called under that lock.

This may not introduce bugs with current notifier implementations (I
didn't check), but it does lessen flexibility?