Re: [PATCH] hugetlb: avoid gratuitous BUG_ON in hugetlb_fault() ->hugetlb_cow()

[Hugh Dickins]

On Sun, 29 Apr 2012, Chris Metcalf wrote:

No, Chris wrote this version on Mon, 30 Apr 2012, but gmail,
or its intereaction with imap, is too confused to grasp that.

> Commit 66aebce747eaf added code to avoid a race condition by
> elevating the page refcount in hugetlb_fault() while calling
> hugetlb_cow().  However, one code path in hugetlb_cow() includes
> an assertion that the page count is 1, whereas it may now also
> have the value 2 in this path.  Consensus is that this BUG_ON
> has served its purpose, so rather than extending it to cover both
> cases, we just remove it.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

It is rather important that we Cc stable on this, since the
earlier fix triggering this BUG went out in 3.0.29, 3.2.16 and 3.3.3
stable.  Sadly, 3.2.16 was the end of the 3.2 line...

> ---
>  mm/hugetlb.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index cd65cb1..baaad5d 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2498,7 +2498,6 @@ retry_avoidcopy:
>  		if (outside_reserve) {
>  			BUG_ON(huge_pte_none(pte));
>  			if (unmap_ref_private(mm, vma, old_page, address)) {
> -				BUG_ON(page_count(old_page) != 1);
>  				BUG_ON(huge_pte_none(pte));
>  				spin_lock(&mm->page_table_lock);
>  				ptep = huge_pte_offset(mm, address & huge_page_mask(h));
> -- 
> 1.6.5.2
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