linux-kernel - Re: [PATCH] mmap_region: cleanup, remove unneeded file != NULL check

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Date:	Mon, 11 Feb 2008 13:34:05 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <mszeredi@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmap_region: cleanup, remove unneeded file != NULL check

Sorry for delay,

On 02/06, Hugh Dickins wrote:
>
> On Sun, 3 Feb 2008, Oleg Nesterov wrote:
> 
> > mmap_region() checks "file != NULL" when we know "file && vma_merge() == T".
> > Also, swap these if/else branches, imho make the code a bit more readable.
> > 
> > Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
> 
> Acked-with-a-but-by: Hugh Dickins <hugh@...itas.com>
> 
> But my but is this: you can go one step further, it's silly to be
> repeating the "if (correct_wcount) atomic_inc..." in both the if
> and the else clauses.

Ah. Shame on me. Of course, I noticed these 2 correct_wcount's, but
didn't realize we can safely use "inode" after fput(). Not only the
caller should have a reference, vma_merge() requires that vm_file == file,
so fput(file) can't destroy the last reference.

> For several minutes I thought that must indicate we already had a
> bug there.  Eventually I realized not: we need deny_write_access()
> above to test and deny atomically, then once we've merged or linked
> the vma it's securely denying in the vma itself: so before returning
> we need to undo our temporary denial.  A brief comment might be
> worthwhile, perhaps something like
> 
> 	/* Once vma denies write, undo our temporary denial count */
> 	if (correct_wcount)
> 		atomic_inc(&inode->i_writecount);

Thanks Hugh, I'll redo this cleanup.

Oleg.

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