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

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Date:	Fri, 30 Jan 2009 10:14:26 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hugh@...itas.com>
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, 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

---
 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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