linux-kernel - Re: + mm-mmapc-replace-find_vma_prepare-with-clearer-find_vma

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Date:	Wed, 12 Sep 2012 16:25:23 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Hugh Dickins <hughd@...gle.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, mm-commits@...r.kernel.org,
	bhalevy@...ian.com, dhillf@...il.com
Subject: Re: + mm-mmapc-replace-find_vma_prepare-with-clearer-find_vma_links.patch
 added to -mm tree

On Wed, 12 Sep 2012, Hugh Dickins wrote:

> > that we can't fix it all up to use 
> > uninitialized_var() so this is handled the proper way?
> 
> I do prefer to avoid it when we can, some reasons given below.
> 

It's easily silenced with __maybe_unused.  I advocated for using that 
attribute for unusued labels in mm/slab_common.c as well where we now do 
things like

	#ifdef CONFIG_DEBUG_SLAB
	out:
	#endif

repeatedly, but others felt this was better than simply doing

	out: __maybe_unused

Not sure what your preferences are for just adding __maybe_unused to the 
declaration.

> > If CONFIG_BUG exists, then we should support it, right?
> 
> Up to a point, yes: we don't encourage switching it off ("Just say Y").
> I'm reluctant to add a notation just for this discouraged case, but how
> how about this patch - does it actually fix the warning you see, David?
> 
> 
> [PATCH mmotm] mm-mmapc-replace-find_vma_prepare-with-clearer-find_vma_links fix
> 
> Strangely, I can no longer get an uninitialized variable warning out of
> copy_vma(), with or without the BUG() there; but David Rientjes gets it
> when he builds with CONFIG_BUG off, which is understandable.
> 
> uninitialized_var() can be a useful tool, but I do prefer to avoid it:
> partly because it might hide future errors, partly because I misspell
> it, but mainly because the need for it comes and goes so mysteriously.
> 
> Given David's preference for no warning, mine for no uninitialized_var,
> and Linus's for renaming BUG() to I_AM_A_MORON() to discourage its use
> in the first place: copy_vma() seems a prime candidate for returning
> failure to mremap instead of crashing the system.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Tested-by: David Rientjes <rientjes@...gle.com>
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