linux-kernel - Re: [PATCH -V7 05/14] hugetlb: avoid taking i_mmap_mutex in unmap_single

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Message-ID: <20120531052540.GA24855@skywalker.linux.vnet.ibm.com>
Date:	Thu, 31 May 2012 10:55:40 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	dhillf@...il.com, mhocko@...e.cz, akpm@...ux-foundation.org,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH -V7 05/14] hugetlb: avoid taking i_mmap_mutex in
 unmap_single_vma() for hugetlb

On Wed, May 30, 2012 at 06:57:47PM -0700, David Rientjes wrote:
> On Wed, 30 May 2012, Aneesh Kumar K.V wrote:
> 
> > From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
> > 
> > i_mmap_mutex lock was added in unmap_single_vma by 502717f4e ("hugetlb:
> > fix linked list corruption in unmap_hugepage_range()") but we don't use
> > page->lru in unmap_hugepage_range any more.  Also the lock was taken
> > higher up in the stack in some code path.  That would result in deadlock.
> > 
> > unmap_mapping_range (i_mmap_mutex)
> >  -> unmap_mapping_range_tree
> >     -> unmap_mapping_range_vma
> >        -> zap_page_range_single
> >          -> unmap_single_vma
> > 	      -> unmap_hugepage_range (i_mmap_mutex)
> > 
> 
> You should be able to show this with lockdep?

I was not able to get a lockdep report

> 
> > For shared pagetable support for huge pages, since pagetable pages are ref
> > counted we don't need any lock during huge_pmd_unshare.  We do take
> > i_mmap_mutex in huge_pmd_share while walking the vma_prio_tree in mapping.
> > (39dde65c9940c97f ("shared page table for hugetlb page")).
> > 
> 
> I think this should be folded into patch 4, the code you're removing here 
> is just added in that function unnecessarily.
> 

I am removing i_mmap_mutex in this patch. That is not added in patch 4.

-aneesh

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