Re: [PATCH] remove unused line for mmap_region()

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Sun, 21 Jun 2009, Huang Shijie wrote:

> 	The variable pgoff is not used in the following codes.
> 	So, just remove the line.
> 
> Signed-off-by: Huang Shijie <shijie8@gmail.com>

Hmm, hmm, well, I suppose a grudging Ack, though I'd happily be overruled.

Of course you are right, so why am I so reluctant to acknowledge it?

Because it's exceptional for addr and pgoff to be different after
coming back from the file's ->mmap method, and if someone adds a
use for pgoff lower down (there was, of course, a use for pgoff
in vma_merge lower down, until Linus moved that higher in 2.6.29),
it's a fair bet that they'll forget to restore the update you're
removing, and a fair bet that nobody will notice that it's gone
wrong for a while.

However, what would be likely to need pgoff lower down, other than
another attempt at vma_merge?  And given how we're now working with
the vma_merge higher up (before pgoff had a chance to be adjusted),
I think we can conclude that anything needing to meddle with pgoff
would be setting some vm_flags that prevent merging anyway.

So I can just about bring myself to Ack your patch, but...

Hugh

> ---
>  mm/mmap.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 34579b2..1dd6aaa 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1210,7 +1210,6 @@ munmap_back:
>  	 *         f_op->mmap method. -DaveM
>  	 */
>  	addr = vma->vm_start;
> -	pgoff = vma->vm_pgoff;
>  	vm_flags = vma->vm_flags;
>  
>  	if (vma_wants_writenotify(vma))
> -- 
> 1.6.0.6

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