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

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Message-ID: <20171214130435.GL16951@dhcp22.suse.cz>
Date:   Thu, 14 Dec 2017 14:04:35 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     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 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?

> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index ec39f73..3d445ee 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -188,7 +188,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
>                                         }
>  
>                                         /* huge pmd was handled */
> -                                       continue;
> +                                       goto next;
>                                 }
>                         }
>                         /* fall through, the trans huge pmd just split */
> @@ -196,6 +196,8 @@ 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;
> +next:
> +               cond_resched();
>         } while (pmd++, addr = next, addr != end);
>  
>         if (mni_start)

-- 
Michal Hocko
SUSE Labs