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

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Date:	Wed, 30 May 2012 18:57:47 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.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, 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?

> 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.
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