linux-kernel - Re: 2.6.23 regression: accessing invalid mmap'ed memory from gdb causes unkillable spinning

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Date:	Thu, 1 Nov 2007 09:08:45 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Duane Griffin <duaneg@...da.com>,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable@...nel.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: 2.6.23 regression: accessing invalid mmap'ed memory from gdb
 causes unkillable spinning

On Thu, 1 Nov 2007, Nick Piggin wrote:
> 
> Untested patch follows

Ok, this looks ok.

Except I would remove the VM_MAYSHARE bit from the test.

That whole bit should go, in fact.

We used to make it something different: iirc, a read-only SHARED mapping 
was downgraded to a non-shared mapping, because we wanted to avoid some of 
the costs we used to have with the VM implementation (actually, I think it 
was various filesystems that don't like shared mappings because they don't 
have a per-page writeback). But we left the VM_MAYSHARE bit on, to get 
/proc/<pid>/mmap things right.

Or something like that. I forget the details. But I *think* we don't 
actually need this any more.

But basically, the "right" way to test for shared mappings is historically 
to just test the VM_MAYSHARE bit - but not *both* bits. Because VM_SHARE 
may have been artificially cleared.

Somebody should double-check my memory.

		Linus
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