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

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Date:	Thu, 29 Jan 2009 20:49:52 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Lee Schermerhorn <Lee.Schermerhorn@...com>
cc:	Hugh Dickins <hugh@...itas.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Maksim Yevmenkin <maksim.yevmenkin@...il.com>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, will@...wder-design.com,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED
 file segments



On Thu, 29 Jan 2009, Lee Schermerhorn wrote:
> 
> Just want to note that install_special_mappings()--used for, e.g., the
> vdso--sets VM_DONTEXPAND [one of the VM_SPECIAL flags] which, if we want
> to prevent merging, makes a lot of sense to me.

Yes. VM_DONTEXPAND in many ways really fits the "don't merge" thing, 
because the whole mremap() VM expansion thing is really equivalent to this 
explicit merge.

> It appears that get_user_pages() will balk at addresses in vmas with 
> VM_IO [and VM_PFNMAP] which might not be what one wants.  For example, 
> you can't pre-populate the ptes via make_pages_present() with this flag.

Well, anybody who plays games with vm_pgoff really _should_ be VM_IO. I 
don't see any real reason for it except for having a very special IO 
mapping. Of course, you quite possibly _also_ want VM_DONTEXPAND.

		Linus
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