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

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Date:   Thu, 8 Feb 2018 20:47:35 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kirill.shutemov@...ux.intel.com, gavin.dg@...ux.alibaba.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: fix potential clearing to referenced flag in
 page_idle_clear_pte_refs_one()



On 2/8/18 8:33 PM, Kirill A. Shutemov wrote:
> On Thu, Feb 08, 2018 at 02:39:26PM -0800, Andrew Morton wrote:
>> On Tue,  6 Feb 2018 08:06:36 +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.
>>>
>>> So, here just break pvmw walk once referenced PTE is found if the page
>>> is a part of THP.
>>>
>>> ...
>>>
>>> --- a/mm/page_idle.c
>>> +++ b/mm/page_idle.c
>>> @@ -67,6 +67,14 @@ static bool page_idle_clear_pte_refs_one(struct page *page,
>>>   		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.
>>> +			 */
>>> +			if (referenced && PageTransCompound(pvmw.page)) {
>>> +				page_vma_mapped_walk_done(&pvmw);
>>> +				break;
>>> +			}
>> This means that the function will no longer clear the referenced bits
>> in all the ptes.  What effect does this have and should we document
>> this in some fashion?
> Yeah, the patch is wrong. We need to get all ptes for THP cleared.
>
> What about something like this instead (untested):

Thanks, Kirill. It looks correct. All ptes should be cleared.

I'm going to prepare v2 patch.

Regards,
Yang

>
> diff --git a/mm/page_idle.c b/mm/page_idle.c
> index 0a49374e6931..6876522c9dce 100644
> --- a/mm/page_idle.c
> +++ b/mm/page_idle.c
> @@ -65,10 +65,10 @@ 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,
> +                       referenced |= ptep_clear_young_notify(vma, addr,
>                                          pvmw.pte);
>                  } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> -                       referenced = pmdp_clear_young_notify(vma, addr,
> +                       referenced |= pmdp_clear_young_notify(vma, addr,
>                                          pvmw.pmd);
>                  } else {
>                          /* unexpected pmd-mapped page? */