Question on i_mmap_mutex locking rule

Question on i_mmap_mutex locking rule

[Aneesh Kumar K.V]

Hi,

I started looking at unmap_hugepage_range. unmap_hugepage_range takes
i_mmap_mutex. But then the same lock is also taken higher up in the
stack. ie via the below chain

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)

But there are other code path that doesn't take i_mmap_mutex. For ex:
-> madvise_dontneed
   -> zap_page_range
      -> unmap_vmas
         -> unmap_single_vma
            -> unmap_page_range
      

similarly unmap_region also don't take i_mmap_mutex. Don't we need to
ensure that we take i_mmap_mutex when we unmap a file backed range ?

-aneesh

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Re: Question on i_mmap_mutex locking rule

[KAMEZAWA Hiroyuki]

(2012/04/02 21:04), Aneesh Kumar K.V wrote:

> 
> Hi,
> 
> I started looking at unmap_hugepage_range. unmap_hugepage_range takes
> i_mmap_mutex. But then the same lock is also taken higher up in the
> stack. ie via the below chain
> 
> 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)
> 


why not deadlock ?

> But there are other code path that doesn't take i_mmap_mutex. For ex:
> -> madvise_dontneed
>    -> zap_page_range
>       -> unmap_vmas
>          -> unmap_single_vma
>             -> unmap_page_range
>       
> 


IIUC, DONTNEED will not change vma layout...so it doesn't require
i_mmap_mutex.

Thanks,
-Kame 


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