linux-kernel - Re: [PATCH 1/5] mmap: don't assume f_op->mmap() doesn't change vma->vm

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Date:	Wed, 15 Apr 2009 11:22:23 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Hugh Dickins <hugh@...itas.com>
CC:	linux-kernel@...r.kernel.org, fuse-devel@...ts.sourceforge.net,
	miklos@...redi.hu, akpm@...ux-foundation.org, npiggin@...e.de
Subject: Re: [PATCH 1/5] mmap: don't assume f_op->mmap() doesn't change vma->vm_file

Hello,

Hugh Dickins wrote:
> On Tue, 14 Apr 2009, Tejun Heo wrote:
> 
>> mmap_region() assumes that vma->vm_file isn't changed by f_op->mmap()
>> and continues to use cache file after f_op->mmap() returns.
> 
> It does use "file" again in the unmap_and_free_vma error path
> (isn't that reasonable? if the ->mmap failed, it shouldn't have
> mucked with vma; and even if it has, then we'd better not change
> the current behaviour of which to fput), but I don't see where else.
> 
> Further down, covering both vma->vm_file previously set and previously
> unset cases, there is a "file = vma->vm_file;" before file is used.
> So I think this patch is not necessary - if it is necessary, it's
> already a bug, because already we switch from /dev/zero to a
> shmem file there.

Right, ->mmap() shouldn't modify @vma if it has failed.  I was trying
to clarify that @vma->file may change as the code was a bit confusing
regarding whether @vma->vm_file is allowed to be substituted or not.
Hmmmm... how about adding a BUG_ON() or WARN_ON() in the failure path
to make sure @vma->vm_file hasn't changed?

Anyways, I'm dropping this patch and updating FUSE mmap implementation
accordingly.  Thanks for the comment.

-- 
tejun
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