linux-kernel - Re: [PATCH v2] mm: thp: fix potential clearing to referenced flag in page_idle_clear_pte_refs

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Date:   Fri, 9 Feb 2018 12:43:00 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     kirill.shutemov@...ux.intel.com, gavin.dg@...ux.alibaba.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: thp: fix potential clearing to referenced flag
 in page_idle_clear_pte_refs_one()

On Sat, 10 Feb 2018 03:12:01 +0800 Yang Shi <yang.shi@...ux.alibaba.com> wrote:

> For PTE-mapped THP, the compound THP has not been split to normal 4K
> pages yet, the whole THP is considered referenced if any one of sub
> page is referenced.
> 
> When walking PTE-mapped THP by pvmw, all relevant PTEs will be checked
> to retrieve referenced bit. But, the current code just returns the
> result of the last PTE. If the last PTE has not referenced, the
> referenced flag will be cleared.
> 
> Just did logical OR for referenced to get the correct result.
> 
> Reported-by: Gang Deng <gavin.dg@...ux.alibaba.com>
> Suggested-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
> v2: adopted the suggestion from Kirill. Not use "||=" style to keep checkpatch
> quiet, otherwise it reports ERROR: spaces required around that '||'
> 
>  mm/page_idle.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_idle.c b/mm/page_idle.c
> index 0a49374..a4baec9 100644
> --- a/mm/page_idle.c
> +++ b/mm/page_idle.c
> @@ -65,11 +65,15 @@ static bool page_idle_clear_pte_refs_one(struct page *page,
>  	while (page_vma_mapped_walk(&pvmw)) {
>  		addr = pvmw.address;
>  		if (pvmw.pte) {
> -			referenced = ptep_clear_young_notify(vma, addr,
> -					pvmw.pte);
> +			/*
> +			 * For PTE-mapped THP, one sub page is referenced,
> +			 * the whole THP is referenced.
> +			 */
> +			referenced = referenced || ptep_clear_young_notify(vma,
> +					addr, pvmw.pte);

That doesn't work.  If `referenced' is already true,
ptep_clear_young_notify() will not be called.

	if (ptep_clear_young_notify(...))
		referenced = true;

would suit.


It makes me wonder what difference this bug makes and why that
difference was not noted in your testing.  Any theories about that? 
How can we design a test which *will* make this error apparent?