linux-kernel - Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM

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Date:	Fri, 30 Jan 2009 18:30:59 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Greg KH <gregkh@...e.de>,
	Maksim Yevmenkin <maksim.yevmenkin@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	will@...wder-design.com, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED
 file segments

On Fri, 30 Jan 2009, Linus Torvalds wrote:
> On Fri, 30 Jan 2009, Hugh Dickins wrote:
> > 
> > ... what I think you have done is break the vma merging on
> > ordinary files: because of that irritating VM_CAN_NONLINEAR
> > flag which generic_file_mmap() and some others add in.
> 
> Ahh. Yes. VM_CAN_NONLINEAR is a "reverse flag", ie unlike the other flags 
> it's to some degree about being extra _normal_, not about being odd. Most 
> special flags tend to disable the VM from doing some clever thing, this 
> one enables it.
> 
> > To break the merging won't cause anyone much trouble,
> > but is a slight regression we should fix.
> 
> Yeah. Just masking it off when comparing is probably the simplest option. 
> Make it a separate #define just for readability. There might be other 
> flags like this in the future.
> 
> > I'd have been very upset not to find something ;)
> 
> Yay for you ;)
> 
> And yes, this is the kind of thing that probably does mean that we're 
> better off with the no-semantic-changes patches in -stable.
> 
> 		Linus

Yes, your patch looks just right to me - or maybe add a comment that
it's only mmap_region()'s call to vma_merge() which needs that mask?

If you hear no more from me, that'll be all I found.

Hugh

> 
> ---
>  mm/mmap.c |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d3fa10a..c581df1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -658,6 +658,9 @@ again:			remove_next = 1 + (end > next->vm_end);
>  	validate_mm(mm);
>  }
>  
> +/* Flags that can be inherited from an existing mapping when merging */
> +#define VM_MERGEABLE_FLAGS (VM_CAN_NONLINEAR)
> +
>  /*
>   * If the vma has a ->close operation then the driver probably needs to release
>   * per-vma resources, so we don't attempt to merge those.
> @@ -665,7 +668,7 @@ again:			remove_next = 1 + (end > next->vm_end);
>  static inline int is_mergeable_vma(struct vm_area_struct *vma,
>  			struct file *file, unsigned long vm_flags)
>  {
> -	if (vma->vm_flags != vm_flags)
> +	if ((vma->vm_flags ^ vm_flags) & ~VM_MERGEABLE_FLAGS)
>  		return 0;
>  	if (vma->vm_file != file)
>  		return 0;
--
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