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: <87y5nrmacr.fsf@skywalker.in.ibm.com>
Date:	Wed, 13 Jun 2012 22:13:00 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Michal Hocko <mhocko@...e.cz>
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

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. 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.

-aneesh

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