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SubjectRe: [PATCH -V8 05/16] hugetlb: avoid taking i_mmap_mutex in unmap_single_vma() for hugetlb
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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

>
> 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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