Re: [PATCH] mm: thp: fix potential clearing to referenced flag in page_idle_clear_pte_refs_one()

[Yang Shi <yang.shi@linux.alibaba.com>]

On 2/9/18 12:16 AM, Kirill A. Shutemov wrote:
> On Thu, Feb 08, 2018 at 08:47:35PM -0800, Yang Shi wrote:
>>
>> 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@linux.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.
> Note, it should be ||=, not |= (although it would work correctly too).

checkpatch doesn't like "||=", it reports no space error. Use referenced 
= referenced || ptep_clear_young_notify(...) to make checkpatch happy.

Regards,
Yang

>
> I should really wake up properly before touching code. :-/
>

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