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

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Date:	Fri, 30 Jan 2009 20:33:46 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Lee Schermerhorn <Lee.Schermerhorn@...com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>,
	Maksim Yevmenkin <maksim.yevmenkin@...il.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, Lee Schermerhorn wrote:
> 
> I tried this patch atop 29-rc3 + your patch from yesterday with my
> simple test program at http://free.linux/hp.com/~lts/Tests/mmap_lock.c. 
> 
> The test program shows the /proc/<pid>/maps before and after the mmap
> and attempted merge.  It's not merging:
> 
> 7fd20a668000-7fd20a66a000 rw-p 7fd20a668000 00:00 0 
> 7fd20a68a000-7fd20a68b000 r--s 00000000 68:23 6608852                    /tmp/tmpfVx1SFL (deleted)
> 7fd20a68b000-7fd20a68c000 r--s 00001000 68:23 6608852                    /tmp/tmpfVx1SFL (deleted)
> 7fd20a68c000-7fd20a690000 rw-p 7fd20a68c000 00:00 0 

Some testing - now that's a great idea!  Good spotting.

> 
> Ad hoc instrumentation shows that it's the VM_ACCOUNT flag that is
> different between the existing file segment and the one attempting the
> merge:
> 
> is_mergeable_vma: !mergable: vma flags:  0x80020f9:0x1020f9
>                                            |         |-VM_ACCOUNT
>                                            +-----------VM_CAN_NONLINEAR

Sorry, I should have noticed that: VM_ACCOUNT was certainly on my
mind as a flag that gives trouble when merging vmas, but I'd
misconvinced myself that it wasn't a problem in this case.

> 
> So happens, I'm mapping with MAP_SHARED, so the VM_ACCOUNT flag gets
> cleared later in mmap_region().  Comments say that this is for checking
> memory availability during shmem_file_setup().  Maybe we can move the
> temporary setting of VM_ACCOUNT until just before the call to
> shmem_zero_setup()?

Please let me think on that - we can live with the status quo for
the moment.  I need to remind myself why I used that peculiar way
of conveying info from mmap.c to shmem.c: the right answer may be
just to pass another arg to shmem_zero_setup and shmem_file_setup.

Hugh
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