Date: Tue, 19 Oct 2004 12:35:22 -0200
From: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: bgagnon@coradiant.com,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Memory leak in 2.4.27 kernel, using mmap raw packet sockets
Message-ID: <20041019143522.GA8688@logos.cnet>
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On Sun, Oct 17, 2004 at 03:39:26AM +0100, Alan Cox wrote:
> On Gwe, 2004-10-15 at 19:23, Marcelo Tosatti wrote:
> > I prefer doing the "if (PageReserved(page)) put_page_testzero(page)" as
> > you propose instead of changing get_user_pages(), as there are several
> > users which rely on its behaviour.
> > 
> > I have applied your fix to the 2.4 BK tree.
> 
> That isnt sufficient. Consider anything else taking a reference to the
> page and the refcount going negative. 

You mean not going negative? The problem here as I understand here is 
we dont release the count if the PageReserved is set, but we should. 

You mean there are other codepaths which release pages? That use __free_pages
which ignores PageReserved pages.

Is the problem wider than what I think?

> And yes 2.6.x has this problem and
> far worse in some ways, but it also has the mechanism to fix it.
> 
> 2.6.x uses VM_IO as a VMA flag which tells the kernel two things
> a) get_user_pages fails on it
> b) core dumping of it is forbidden
> 
> 2.6.x is missing a whole pile of these (fixed in the 2.6.9-ac tree I'm
> putting together). I *think* remap_page_range() in 2.6.x can just set
> VM_IO, but older kernels didn't pass the vma so all the users would need
> fixing (OSS audio, media/video, usb audio, usb video, frame buffer
> etc).

All these are have codepaths which release pages using put_page()'s? 

Thanks
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