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

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Date:	Sun, 14 Oct 2007 09:44:08 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	Ryan Finnie <ryan@...nie.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Erez Zadok <ezk@...sunysb.edu>, 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

On Sat, 13 Oct 2007, Pekka Enberg wrote:
> On 10/12/07, Hugh Dickins <hugh@...itas.com> wrote:
> > But I keep suspecting that the answer might be the patch below (which
> > rather follows what drivers/block/rd.c is doing).  I'm especially
> > worried that, rather than just AOP_WRITEPAGE_ACTIVATE being returned
> > to userspace, bad enough in itself, you might be liable to hit that
> > BUG_ON(page_mapped(page)).  shmem_writepage does not expect to be
> > called by anyone outside mm/vmscan.c, but unionfs can now get to it?
> 
> Doesn't msync(2) get to it via mm/page-writeback.c:write_cache_pages()
> without unionfs even?

I believe not.  Please do double-check my assertions, I've always found
the _writepages paths rather twisty; but my belief (supported by the
fact that we've not hit shmem_writepage's BUG_ON(page_mapped(page))
in five years) is that tmpfs/shmem opts out of all of that with its
	.capabilities	= BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK,
in shmem_backing_dev_info, which avoids all those _writepages avenues
(e.g. through bdi_cap_writeback_dirty tests), and write_cache_pages is
just a subfunction of the _writepages.

So, while I don't disagree with your patch to write_cache_pages (though
it wasn't clear to me whether it should break from or continue the loop
if it ever does meet an AOP_WRITEPAGE_ACTIVATE), I don't think that's
really the root of the problem.

Hugh
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