linux-kernel - Re: [PATCH -V8 05/16] hugetlb: avoid taking i_mmap_mutex in unmap_single

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Date:	Sat, 09 Jun 2012 18:33:05 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	dhillf@...il.com, rientjes@...gle.com, mhocko@...e.cz,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH -V8 05/16] hugetlb: avoid taking i_mmap_mutex in unmap_single_vma() for hugetlb

Johannes Weiner <hannes@...xchg.org> writes:

> On Sat, Jun 09, 2012 at 02:29:50PM +0530, 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)
>> 
>> 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@...ux.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

>
> 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

-aneesh

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