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

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Date:	Tue, 19 Dec 2006 16:23:08 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Andrei Popa <andrei.popa@...eo.ro>, Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hugh@...itas.com>,
	Florian Weimer <fw@...eb.enyo.de>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	Martin Michlmayr <tbm@...ius.com>
Subject: Re: 2.6.19 file content corruption on ext3



On Wed, 20 Dec 2006, Peter Zijlstra wrote:
> On Mon, 2006-12-18 at 12:14 -0800, Linus Torvalds wrote:
> > OR:
> > 
> >  - page_mkclean_one() is simply buggy.
> 
> GOLD!

Ok. I was looking at that, and I wondered..

However, if that works, then I _think_ the correct sequence is the 
following..

The rule should be:
 - we flush the tlb _after_ we have cleared it, but _before_ we insert the 
   new entry.

But I dunno. These things are damn subtle. Does this patch fix it for you?

I actually suspect we should do this as an arch-specific macro, and 
totally replace the current "ptep_clear_flush_dirty()" with one that does 
"ptep_clear_flush_dirty_and_set_wp()".

Because what I'd _really_ prefer to do on x86 (and probably on most other 
sane architectures) is to do

 - atomically replace the pte with the EXACT SAME ONE, but one that 
   has the writable bit clear.

	bit_clear(_PAGE_BIT_RW, &(ptep)->pte_low);

 - flush the TLB, making sure that all CPU's will no longer write to it:

	flush_tlb_page(vma, address);

 - finally, just fetch-and-clear the dirty bit (and since it's no longer 
   writable, nobody should be settign it any more)

	ret = bit_clear(__PAGE_BIT_DIRTY, &(ptep)->pte_low);

and now we should be all done.

But the "ptep_get_and_clear() + flush_tlb_page()" sequence should 
hopefully also work.

Pls test.

		Linus

----
diff --git a/mm/rmap.c b/mm/rmap.c
index d8a842a..eec8706 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -448,9 +448,10 @@ static int page_mkclean_one(struct page *page, struct vm_area_struct *vma)
 		goto unlock;
 
 	entry = ptep_get_and_clear(mm, address, pte);
+	flush_tlb_page(vma, address);
 	entry = pte_mkclean(entry);
 	entry = pte_wrprotect(entry);
-	ptep_establish(vma, address, pte, entry);
+	set_pte_at(mm, address, pte, entry);
 	lazy_mmu_prot_update(entry);
 	ret = 1;
 

-
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