linux-kernel - Re: msync(2) bug(?), returns AOP_WRITEPAGE

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Message-Id: <200710161802.l9GI2ca6012758@agora.fsl.cs.sunysb.edu>
Date:	Tue, 16 Oct 2007 14:02:38 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	"Pekka Enberg" <penberg@...helsinki.fi>
Cc:	"Erez Zadok" <ezk@...sunysb.edu>,
	"Hugh Dickins" <hugh@...itas.com>, "Ryan Finnie" <ryan@...nie.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	cjwatson@...ntu.com, linux-mm@...ck.org
Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland 

In message <84144f020710150447o94b1babo8b6e6a647828465f@...l.gmail.com>, "Pekka Enberg" writes:
> Hi,
> 
> On 10/15/07, Erez Zadok <ezk@...sunysb.edu> wrote:
> > Pekka, with a small change to your patch (to handle time-based cache
> > coherency), your patch worked well and passed all my tests.  Thanks.
> >
> > So now I wonder if we still need the patch to prevent AOP_WRITEPAGE_ACTIVATE
> > from being returned to userland.  I guess we still need it, b/c even with
> > your patch, generic_writepages() can return AOP_WRITEPAGE_ACTIVATE back to
> > the VFS and we need to ensure that doesn't "leak" outside the kernel.
> 
> I wonder whether _not setting_ BDI_CAP_NO_WRITEBACK implies that
> ->writepage() will never return AOP_WRITEPAGE_ACTIVATE for
> !wbc->for_reclaim case which would explain why we haven't hit this bug
> before. Hugh, Andrew?
> 
> And btw, I think we need to fix ecryptfs too.

Yes, ecryptfs needs this fix too (and probably a couple of other mmap fixes
I've made to unionfs recently -- Mike Halcrow already knows :-)

Of course, running ecryptfs on top of tmpfs is somewhat odd and uncommon;
but with unionfs, users use tmpfs as the copyup branch very often.

>                                            Pekka

Erez.
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