linux-kernel - Re: [PATCH V2] mm/mprotect: Add a cond_resched() inside change_pmd

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Message-Id: <988a79ce-2803-85e7-d810-b4b2d4ba6b26@linux.vnet.ibm.com>
Date:   Thu, 14 Dec 2017 19:00:36 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH V2] mm/mprotect: Add a cond_resched() inside
 change_pmd_range()

On 12/14/2017 06:57 PM, Michal Hocko wrote:
> On Thu 14-12-17 18:50:41, Anshuman Khandual wrote:
>> On 12/14/2017 06:34 PM, Michal Hocko wrote:
>>> On Thu 14-12-17 18:25:54, Anshuman Khandual wrote:
>>>> On 12/14/2017 04:59 PM, Michal Hocko wrote:
>>>>> On Thu 14-12-17 16:44:26, Anshuman Khandual wrote:
>>>>>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>>>>>> index ec39f73..43c29fa 100644
>>>>>> --- a/mm/mprotect.c
>>>>>> +++ b/mm/mprotect.c
>>>>>> @@ -196,6 +196,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
>>>>>>  		this_pages = change_pte_range(vma, pmd, addr, next, newprot,
>>>>>>  				 dirty_accountable, prot_numa);
>>>>>>  		pages += this_pages;
>>>>>> +		cond_resched();
>>>>>>  	} while (pmd++, addr = next, addr != end);
>>>>>>  
>>>>>>  	if (mni_start)
>>>>> this is not exactly what I meant. See how change_huge_pmd does continue.
>>>>> That's why I mentioned zap_pmd_range which does goto next...
>>>> I might be still missing something but is this what you meant ?
>>> yes, except
>>>
>>>> Here we will give cond_resched() cover to the THP backed pages
>>>> as well.
>>> but there is still 
>>> 		if (!is_swap_pmd(*pmd) && !pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)
>>> 				&& pmd_none_or_clear_bad(pmd))
>>> 			continue;
>>>
>>> so we won't have scheduling point on pmd holes. Maybe this doesn't
>>> matter, I haven't checked but why should we handle those differently?
>>
>> May be because it is not spending much time for those entries which
>> can really trigger stalls, hence they dont need scheduling points.
>> In case of zap_pmd_range(), it was spending time either in
>> __split_huge_pmd() or zap_huge_pmd() hence deserved a scheduling point.
> 
> As I've said, I haven't thought much about that but the discrepancy just
> hit my eyes. So if there is not a really good reason I would rather use
> goto next consistently.

Sure, will respin with the changes.