linux-kernel - Re: [PATCH -V9 04/15] hugetlb: use mmu_gather instead of a temporary linked list for accumulating pages

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Message-ID: <20120614071423.GA27397@tiehlicka.suse.cz>
Date:	Thu, 14 Jun 2012 09:14:23 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	dhillf@...il.com, rientjes@...gle.com, akpm@...ux-foundation.org,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH -V9 04/15] hugetlb: use mmu_gather instead of a temporary
 linked list for accumulating pages

On Wed 13-06-12 22:13:00, Aneesh Kumar K.V wrote:
> Michal Hocko <mhocko@...e.cz> writes:
> 
> > On Wed 13-06-12 16:59:23, Michal Hocko wrote:
> >> On Wed 13-06-12 15:57:23, Aneesh Kumar K.V wrote:
> >> > From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
> >> > 
> >> > Use a mmu_gather instead of a temporary linked list for accumulating
> >> > pages when we unmap a hugepage range
> >> 
> >> Sorry for coming up with the comment that late but you owe us an
> >> explanation _why_ you are doing this.
> >> 
> >> I assume that this fixes a real problem when we take i_mmap_mutex
> >> already up in 
> >> unmap_mapping_range
> >>   mutex_lock(&mapping->i_mmap_mutex);
> >>   unmap_mapping_range_tree | unmap_mapping_range_list 
> >>     unmap_mapping_range_vma
> >>       zap_page_range_single
> >>         unmap_single_vma
> >> 	  unmap_hugepage_range
> >> 	    mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);
> >> 
> >> And that this should have been marked for stable as well (I haven't
> >> checked when this has been introduced).
> >> 
> >> But then I do not see how this help when you still do this:
> >> [...]
> >> > diff --git a/mm/memory.c b/mm/memory.c
> >> > index 1b7dc66..545e18a 100644
> >> > --- a/mm/memory.c
> >> > +++ b/mm/memory.c
> >> > @@ -1326,8 +1326,11 @@ static void unmap_single_vma(struct mmu_gather *tlb,
> >> >  			 * Since no pte has actually been setup, it is
> >> >  			 * safe to do nothing in this case.
> >> >  			 */
> >> > -			if (vma->vm_file)
> >> > -				unmap_hugepage_range(vma, start, end, NULL);
> >> > +			if (vma->vm_file) {
> >> > +				mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);
> >> > +				__unmap_hugepage_range(tlb, vma, start, end, NULL);
> >> > +				mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
> >> > +			}
> >> >  		} else
> >> >  			unmap_page_range(tlb, vma, start, end, details);
> >> >  	}
> >
> > Ahhh, you are removing the lock in the next patch. Really confusing and
> > not nice for the stable backport.
> > Could you merge those two patches and add Cc: stable? 
> > Then you can add my
> > Reviewed-by: Michal Hocko <mhocko@...e.cz>
> >
> 
> In the last review cycle I was asked to see if we can get a lockdep
> report for the above and what I found was we don't really cause the
> above deadlock with the current codebase because for hugetlb we don't
> directly call unmap_mapping_range. 

Ahh, ok I missed that.

> But still it is good to remove the i_mmap_mutex, because we don't need
> that protection now. I didn't mark it for stable because of the above
> reason.

Thanks for clarification

> 
> -aneesh
> 

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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