linux-kernel - Re: 2.6.19 file content corruption on ext3

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Date:	Thu, 28 Dec 2006 15:01:29 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	David Miller <davem@...emloft.net>
cc:	akpm@...l.org, guichaz@...oo.fr, ranma@...edrich.de,
	gordonfarquharson@...il.com, mh+linux-kernel@...schlus.de,
	nickpiggin@...oo.com.au, andrei.popa@...eo.ro,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	hugh@...itas.com, fw@...eb.enyo.de, tbm@...ius.com,
	arjan@...radead.org, kenneth.w.chen@...el.com
Subject: Re: 2.6.19 file content corruption on ext3



On Thu, 28 Dec 2006, David Miller wrote:
> 
> What happens when we writeback, to the PTEs?

Not a damn thing.

We clear the PTE's _before_ we even start the write. The writeback does 
nothing to them. If the user dirties the page while writeback is in 
progress, we'll take the page fault and re-dirty it _again_.

> page_mkclean_file() iterates the VMAs and when it finds a shared
> one it goes:
> 
> 		entry = ptep_clear_flush(vma, address, pte);
> 		entry = pte_wrprotect(entry);
> 		entry = pte_mkclean(entry);
> 
> and that's fine, but that PTE is still marked writable, and
> I think that's key.

No it's not. It's right there. "pte_wrprotect(entry)". You even copied it 
yourself.

> What does the fault path do in this situation?
> 
> 	if (write_access) {
> 		if (!pte_write(entry))
> 			return do_wp_page(mm, vma, address,
> 					pte, pmd, ptl, entry);

So we call "do_wp_page()", and that does everythign right.

		Linus
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