linux-kernel - Re: [PATCH] sys_remap_file_pages: fix ->vm

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Date:	Wed, 6 Feb 2008 20:33:07 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	Matt Helsley <matthltc@...ibm.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	William Lee Irwin III <wli@...omorphy.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] sys_remap_file_pages: fix ->vm_file accounting

On Sun, 3 Feb 2008, Oleg Nesterov wrote:
> 
> So I have to try to find another bug ;) Suppose that ->load_binary() does
> a series of do_mmap(MAP_EXECUTABLE). It is possible that mmap_region() can
> merge 2 vmas. In that case we "leak" ->num_exe_file_vmas. Unless I missed
> something, mmap_region() should do removed_exe_file_vma() when vma_merge()
> succeds (near fput(file)).

Or there's the complementary case of a VM_EXECUTABLE vma being
split in two, for example by an mprotect of a part of it.

Sorry, Matt, I don't like your patch at all.  It seems to add a fair
amount of ugliness and unmaintainablity, all for a peculiar MVFS case
(you've tried to argue other advantages, but not always convinced!).

And I found it quite hard to see where the crucial difference comes.
I guess it's that MVFS changes vma->vm_file in its ->mmap?  Well, if
MVFS does that, maybe something else does that too, but precisely to
rely on the present behaviour of /proc/pid/exe - so in fixing for
MVFS, we'd be breaking that hypothetical other?

I can understand patches to avoid mmap_sem for /proc/pid/exe, but
this one just seems too messy for too special an out-of-tree case.
(I've no last word on this, but that's my opinion.)

Hugh
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