Date: Sat, 9 Jun 2012 16:49:18 +0200
From: Johannes Weiner <hannes@cmpxchg.org>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: linux-mm@kvack.org, kamezawa.hiroyu@jp.fujitsu.com, dhillf@gmail.com,
        rientjes@google.com, mhocko@suse.cz, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH -V8 05/16] hugetlb: avoid taking i_mmap_mutex in
 unmap_single_vma() for hugetlb
Message-ID: <20120609144918.GI1761@cmpxchg.org>
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On Sat, Jun 09, 2012 at 06:33:05PM +0530, Aneesh Kumar K.V wrote:
> Johannes Weiner <hannes@cmpxchg.org> writes:
> 
> > On Sat, Jun 09, 2012 at 02:29:50PM +0530, Aneesh Kumar K.V wrote:
> >> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.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)
> >> 
> >> 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")).
> >> 
> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> >
> > This patch (together with the previous one) seems like a bugfix that's
> > not really related to the hugetlb controller, unless I miss something.
> >
> > Could you please submit the fix separately?
> 
> Patches upto 6 can really got in a separate series. I was not sure
> whether I should split them. I will post that as a separate series now

Ok, thanks, that will make it easier to upstream the controller.

> > Maybe also fold the two patches into one and make it a single bugfix
> > change that gets rid of the lock by switching away from page->lru.
> 
> I wanted to make sure the patch that drop i_mmap_mutex is a separate one
> so that we understand and document the locking details separately

Nothing prevents you from writing a proper changelog :-) But changing
from page->lru to an on-stack array does not have any merit by itself,
so it just seems like a needless dependency between two patches that
fix one problem (pita for backports into stable/distro kernels).
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