Re: [PATCH] mm: add pte_present() check on existing hugetlb_entry callbacks

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Fri, Mar 07, 2014 at 01:35:02AM -0500, Naoya Horiguchi wrote:
> On Thu, Mar 06, 2014 at 04:16:29PM -0500, Sasha Levin wrote:
> > On 03/06/2014 11:08 AM, Naoya Horiguchi wrote:
> > > And I found my patch was totally wrong because it should check
> > > !pte_present(), not pte_present().
> > > I'm testing fixed one (see below), and the problem seems not to reproduce
> > > in my environment at least for now.
> > > But I'm not 100% sure, so I need your double checking.
> > 
> > Nope, I still see the problem. Same NULL deref and trace as before.
> 
> Hmm, that's unfortunate.
> I tried to find out how this reproduces and the root cause, but no luck.
> So I suggest to add !PageHuge check before entering isolate_huge_page(),
> which certainly gets over this problem.
> 
> I think "[PATCH] mm: add pte_present() check on existing hugetlb_entry"
> is correct itself although it didn't fix this race.

Andrew, could you consider picking up this patch (below) and "[PATCH] mm:
add pte_present() check on existing hugetlb_entry" (previously posted in
this thread) into linux-mm?

This patch is to be folded into "mempolicy: apply page table walker on
queue_pages_range()," and another one is into "pagewalk: update page
table walker core."

Or do I need to repost them?

Thanks,
Naoya

> Thanks,
> Naoya
> ---
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Date: Fri, 7 Mar 2014 00:59:41 -0500
> Subject: [PATCH] mm/mempolicy.c: add comment in queue_pages_hugetlb()
> 
> We have a race where we try to migrate an invalid page, resulting in
> hitting VM_BUG_ON_PAGE in isolate_huge_page().
> queue_pages_hugetlb() is OK to fail, so let's check !PageHuge before
> queuing it with some comment as a todo reminder.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  mm/mempolicy.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 494f401bbf6c..175353eb7396 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -530,6 +530,17 @@ static int queue_pages_hugetlb(pte_t *pte, unsigned long addr,
>  	if (!pte_present(entry))
>  		return 0;
>  	page = pte_page(entry);
> +
> +	/*
> +	 * TODO: Trinity found that page could be a non-hugepage. This is an
> +	 * unexpected behavior, but it's not clear how this problem happens.
> +	 * So let's simply skip such corner case. Page migration can often
> +	 * fail for various reasons, so it's ok to just skip the address
> +	 * unsuitable to hugepage migration.
> +	 */
> +	if (!PageHeadHuge(page))
> +		return 0;
> +
>  	nid = page_to_nid(page);
>  	if (node_isset(nid, *qp->nmask) == !!(flags & MPOL_MF_INVERT))
>  		return 0;
> -- 
> 1.8.5.3
> 
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